High electron mobility transistor

ABSTRACT

A high electron mobility transistor for analog applications comprising: a substrate; an epitaxial III-N semiconductor layer stack on top of said substrate, said epitaxial III-N semiconductor layer stack comprising: a first active III-N layer; and a second active III-N layer comprising a recess; with a two dimensional Electron Gas in between III-N; a gate on top of said epitaxial III-N semiconductor layer stack; and a passivation stack between said epitaxial III-N semiconductor layer stack and said gate, wherein said passivation stack comprises an electron accepting dielectric layer adapted to deplete said two dimensional Electron Gas when said gate is not biased; wherein said electron accepting dielectric layer extends in said recess and comprises magnesium nitride doped with silicon and/or aluminum.

FIELD OF THE INVENTION

The present invention generally relates to semiconductor devices and,more particularly, to high electron mobility transistors that comprisenitride-based active layers.

BACKGROUND OF THE INVENTION

Semiconductor devices comprising Gallium Nitride, also referred to asGaN, have the ability to carry large currents and to support highvoltages. This makes them increasingly desirable for power semiconductordevices. In the recent years, research efforts focused on developingdevices for high power/high frequency applications. In general, themanufactured devices for these types of applications are based on devicestructures that exhibit high electron mobility and are referred to asheterojunction field effect transistors, also called HFETs, highelectron mobility transistors, also called HEMTs, or modulation dopedfield effect transistors, also called MODFETs. HEMTs are for exampleuseful for analog circuit applications, such as RF/microwave poweramplifiers or power switches. Such devices can typically withstand highvoltages, e.g. up to 1000 Volts, or operate at high frequencies, e.g.from 100 kHz to 100 GHz.

There exist two types of polarization in wurzite III-nitride layer,namely piezoelectricity and spontaneous polarization. Piezoelectricitycan occur if the crystal is non-centrosymmetric and the constituentatoms differ in size and electronegativity. For example, a Wurtzitecrystal or a GaN layer are non-centrosymmetric. Spontaneous polarizationis a special case of the above and occurs if the crystal is distortedfrom its ideal shape, for example due to different size of itsconstituent atoms such as Ga and N atoms.

A GaN-based HEMT comprises at least two nitride layers. The nitridelayers are formed of different materials which have different bandgapsand a different degree of polarisation. The different materials in theadjacent nitride layers cause a discrete step in the polarization andbandgap energy, which contributes to a conductive two dimensionalElectron Gas, also referred to as 2DEG, which allows charge to flowthrough the device and which is located near the junction of the twolayers, and more particularly in the layer with the narrower bandgap. Inthe scientific publication entitled “Polarization effects, surfacestates, and the source of electrons in AlGaN/GaN heterostructure fieldeffect transistors” published on Jul. 10, 2000 in Applied PhysicsLetters Volume 77, number 2, Ibbetson et al. theoretically andexperimentally examine the origin of the 2DEG in AlGaN/GaNheterostructure field effect transistors. The structure comprises thefollowing space charge components: a buffer polarization charge in theGaN layer which is ionic and fixed, a surface charge due to ionizeddonors on top of the AlGaN layer, fixed and ionic polarization-inducedcharges at the AlGaN/GaN interface and the top surface of the AlGaNlayer and a negative charge due to the electrons in the quantum wells inthe 2DEG region. The sum of the various charges is zero since thestructure as a whole must be charge neutral in the absence of anexternally applied field. Ibbetson et al. theoretically andexperimentally demonstrate that the donor-like surface states such asGa-dangling bonds or impurities at the surface of the AlGaN layer arelikely a source of 2DEG electrons in HFETs.

In the scientific publication entitled “Effects of Si deposition onAlGaN barrier surfaces in GaN heterostructure field-effect transistors”published on Jun. 27, 2008, Onojima et al. illustrate schematic modelsthat explain the reduction of the AlGaN potential barrier height and theincrease in the 2DEG density of AlGaN/GaN HFETs due to silicon nitridepassivation, also referred to as SiN passivation. The possible cause forthe reduction of the AlGaN potential barrier height is that the Si atomslocated at the SiN/AlGaN interface act as positively ionized donorswhich can partially neutralize the negative polarization charges of theAlGaN surface and thereby increase the 2DEG density through polarizationeffects.

Because of the 2DEG region existing under the gate at zero gate bias,most nitride devices are normally on, or so-called depletion modedevices. A certain negative voltage on the gate, called thresholdvoltage, is required to deplete the 2DEG through capacitive coupling.For certain applications, such as e.g. power switching, a non-zero gatevoltage to switch off the device is undesired. In such a case, the gatecontrol needs to work in such a way that, if the controlling circuitryfails for whatever reason, there is no galvanic connection between thesource and the drain. If the 2DEG region is depleted, i.e. removed,below the gate at zero applied gate bias, the device can be anenhancement mode device. Enhancement mode devices are normally off andare desirable because of the added safety they provide. An enhancementmode device requires a positive bias applied at the gate in order toconduct current. In particular, a positive voltage is applied to thegate in order to move the 2DEG below the Fermi level. Once anothervoltage is applied between the source and the drain, the electrons inthe 2DEG move from source to drain. In another case, having theopportunity to co-integrate normally-on and normally-off devices allowsthe creation of logic functionality, such as NOT, OR, AND, NOR, NAND andXOR gates. In another case, enhancement mode devices eliminate the needfor a negative supply voltage, thereby reducing circuit complexity andcost.

It can be understood from Onojima et al. that, when passivating thesurface of a Ga-based HEMT with SiN, the Si atoms of the SiN layer giveelectrons to the 2DEG of the HEMT, thereby populating the channel of theHEMT. The crystalline SiN is a continuation of the III-nitride crystaland Si atoms act as donors. In other words, a SiN passivation layer atthe surface of a Ga-based HEMT enhances the 2DEG of the HEMT.Passivating the surface of a HEMT with SiN therefore prevents thedepletion mode of a HEMT and promotes the conductivity of the 2DEG evenat zero gate bias, so it consolidates normally-on operation. On theother hand, for applications using transistors as switches or hightemperature capable integrated circuits, it is desirable to havenormally off devices. Accordingly, there continues to be a need in theart for improved methods and structures for devices capable ofperforming in high power, high voltage, high speed and/or hightemperature conditions.

It is an objective of the present invention to disclose a device thatovercomes the above identified shortcomings of existing solutions. Moreparticularly, it is an objective to disclose a high electron mobilitytransistor comprising an improved passivation layer and exhibiting animproved enhancement mode.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, the above definedobjectives are realized by a high electron mobility transistor foranalog applications, the high electron mobility transistor comprising:

-   -   a substrate;    -   an epitaxial III-N semiconductor layer stack on top of the        substrate, the epitaxial III-N semiconductor layer stack        comprising an active layer, the active layer comprising:        -   a first active III-N layer; and        -   a second active III-N layer comprising a recess in a gate            region; with a two dimensional Electron Gas between the            first active III-N layer and the second active III-N layer;    -   a gate on top of the epitaxial III-N semiconductor layer stack;        and    -   a passivation stack between the epitaxial III-N semiconductor        layer stack and the gate, wherein the passivation stack        comprises an electron accepting dielectric layer adapted to        deplete the two dimensional Electron Gas when the gate is not        biased; wherein the electron accepting dielectric layer extends        in the recess and wherein the electron accepting dielectric        layer comprises magnesium nitride doped with silicon and/or        aluminum.

This way, the high electron mobility transistor of the present inventioncomprises an improved passivation stack which enhances the normally offoperation and thereby improves the enhancement mode of the high electronmobility transistor. Indeed, the passivation stack of the high electronmobility transistor according to the present invention comprises anelectron accepting dielectric layer which creates an acceptor level atthe interface between the passivation stack and the epitaxial III-Nsemiconductor layer stack. The electron accepting dielectric layer ofthe present invention depletes electrons from the two dimensionalElectron Gas, also referred to as 2DEG, when the gate of the highelectron mobility transistor is not biased. No channel is thereforepresent and no current flow occurs until the high electron mobilitytransistor is biased for operation. In particular, in operation, a biasvoltage is applied to the gate of the high electron mobility transistorin order to move the 2DEG from below the Fermi level. Once anothervoltage is applied between the source and the drain of the high electronmobility, the electrons in the 2DEG flow from source to drain. The highelectron mobility transistor according to the present invention istherefore suitable for applications such as e.g. power switching orintegrated logic for which negative polarity gate supply is undesired.The gate polarity of the high electron mobility transistor according tothe present invention is additionally desirable because of the addedsafety it provides.

Because of the recess in the second active III-N layer and in the gateregion, the high electron mobility transistor according to the presentinvention demonstrates a much higher leakage current from the gate tothe 2DEG than a similar high electron mobility transistor which does notcomprise a recess in the second active III-N layer. This is because inthe case of a thinner barrier layer, there is an increased likelihood ofincreased tunneling, trap-assisted tunneling as well as a representinglower barrier height for electrons to overcome and be transportedthrough thermionic emission, or TE and Field-assisted thermionicemission, or FTE. Doping magnesium nitride, or MgN, with silicon, alsoreferred to as Si, and/or with aluminum, also referred to as Al,increases the bandgap of the electron accepting dielectric layer. Whensuch electron accepting dielectric layer extends in the recess in thesecond active III-N layer in the gate region, and between the gate andthe 2DEG, the leakage current is therefore reduced. Additionally, dopingMgN with Si and/or Al increases the dielectric constant of the electronaccepting dielectric layer, thereby allowing a better coupling betweenthe gate and the 2DEG and demonstrating an improved conductance.Additionally, thanks to the recess in the second active III-N layer inthe gate region, the electron accepting dielectric layer is broughtcloser to the 2DEG, thereby improving the effect of depletion ofelectrons from the 2DEG by the electron accepting dielectric layer whenthe gate is not biased. Doping magnesium nitride in the context of theinvention is understood as alloying magnesium nitride with siliconand/or aluminum. In other words, the electron accepting dielectric layeris obtained by impurity doping independently from the method accordingto which it is manufactured. According to the present invention, SiN isalloyed with Al or Mg by introducing a well-controlled flow ofrespectively trimethylaluminium, also referred to as TMA, orbis-cyclo-pentadienyl-magnesium (Cp)₂Mg in an MOCVD chamber during thedeposition of SiN using silane, also referred to as SiH₄ or ammonia,also referred to as NH₃ precursors. In other words, the electronaccepting dielectric layer comprises magnesium nitride alloyed withsilicon and/or aluminum. Alternatively, the electron acceptingdielectric layer comprises silicon nitride alloyed with magnesium andaluminum.

Nitride atoms of the electron accepting dielectric layer bond to GroupIII atoms of the second active III-N layer along the passivation contactinterface. Incorporating Mg atoms in the passivation stack of a highelectron mobility transistor therefore creates an electron acceptinglevel at the interface between the epitaxial III-N semiconductor layerstack and the passivation stack, thereby depleting electrons from the2DEG channel of the high electron mobility transistor when the gate isnot biased. A negative surface charge is provided by the ionized Mgacceptors at the interface between the epitaxial III-N semiconductorlayer stack and the passivation stack.

MgSiN depletes the channel of the high electron mobility transistor whenthe gate of the high electron mobility transistor is not biased andthereby improves the enhancement mode of the high electron mobilitytransistor. The material MgSiN of the electron accepting dielectriclayer exhibits a large or wide bandgap, which makes it an interestingdielectric layer for the gate dielectric of a high electron mobilitytransistor to prevent leakage. The scientific publication of Quirk J. B.et al. entitled “Band gap and electronic structure of MgSiN₂” publishedin Applied Physics Letters Volume 105, Issue 11 in September 2014discloses a bandgap of MgSiN equal to 6.3 eV. Aluminum doping, alsoreferred to as Al doping increases the bandgap of the material of theelectron accepting dielectric layer, which results in an even moreinteresting dielectric layer for the passivation of a high electronmobility transistor and the use as gate dielectric as a higher bandgapwill more effectively block electrons from leaking to or from the gate.Additionally, Al doping influences the etching rate in Fluor-basedplasma of the passivation stack. In other words, doping the passivationstack with Aluminum creates an etch stop layer for Fluor-based plasmaetching. The bandgap of MgAlSiN is expected to be higher than 6 eV.

A two-dimensional electron gas is a gas of electrons free to move in twodimensions, but tightly confined in the first. This tight confinementleads to quantized energy levels for motion in that direction. Theelectrons appear to be a 2D sheet embedded in a 3D world. A device ofparticular interest for high power and/or high frequency applications isthe high electron mobility transistor, also referred to as HEMT.According to the present invention, the passivation stack is formedbetween the epitaxial III-N semiconductor layer stack and the gate. Thepassivation stack may be formed only under the gate and may serveadditionally as gate dielectric. Alternatively, the passivation stackmay be formed on top of the epitaxial III-N semiconductor layer stackand may fully cover the epitaxial III-N semiconductor layer stack.Alternatively, the passivation stack may be formed on top of theepitaxial III-N semiconductor layer stack and partially cover thesurface of the epitaxial III-N semiconductor layer stack, for example itmay be formed in the ungated area between the source and the drain ofthe high mobility electron transistor according to the presentinvention, where it serves as passivation and prevents the depletion ofthe underlying 2DEG.

The bias voltage of the gate of the high electron mobility transistoraccording to the present invention depends on the thickness of theelectron accepting dielectric layer, in particular towards positivevoltages. Indeed, the electron accepting dielectric layer shifts themaximum gate bias of the high electron mobility transistor according tothe present invention to large voltages and also shifts the thresholdvoltage of the high electron mobility transistor. A gate bias voltage ofthe high electron mobility transistor according to the present inventionis comprised between −10 Volts and 20 Volts, preferably between 0 Voltand 10 Volts. A threshold voltage of the high electron mobilitytransistor according to the present invention is comprised between 1Volt and 5 Volts, preferably between 1 Volt and 2 Volts. By contrast,for a normally on high electron mobility transistor, the gate biasvoltage typically reaches −2 Voltages and the gate bias range of thehigh electron mobility transistor is in general comprised between −10Volts and 2 Volts.

The second active III-N layer comprises a recess in a gate region,wherein the recess extends at least partially in said second activeIII-N layer and wherein the electron accepting dielectric layer extendsin the recess such that the passivation surface is in direct contactwith the second passivation surface in the recess.

This way, the high electron mobility transistor according to the presentinvention is an insulated gate HEMT with a recess under the gate formedin the second active III-N layer of the epitaxial III-N semiconductorlayer stack. This way, the threshold voltage of the high electronmobility transistor shifts towards positive voltages and this improvesthe enhancement mode of the high electron mobility transistor.

Embodiments of the present invention may be particularly well suited foruse in nitride-based devices such as Group III-nitride based HEMTs.Group III-nitride, or group III-N, refers to semiconductor compoundsformed between elements in Group III of the periodic table, for exampleBoron, also referred to as B, Aluminum, also referred to as Al, Gallium,also referred to as Ga, Indium, also referred to as In, and Nitrogen,also referred to as N. Example of binary Group III-nitride compounds areGaN, AlN, BN, etc. Group III-nitride also refers to ternary andquaternary compounds such as for example AlGaN and InAlGaN.

Alternatively, the epitaxial III-N semiconductor layer stack comprisesan epitaxially grown buffer layer grown between the substrate and theactive layer. The buffer layer may be of a different nature than thesubstrate, in that for instance the bandgap of the substrate and bufferlayer are relatively far apart (such as 1.1 eV and 6.2 eV respectively),in the sense that the buffer layer has a large bandgap, in order toprovide present characteristics, such as high break down voltage, e.g.larger than 250 V, preferably larger than 500 V, even more preferablylarger than 1000 V, such as larger than 2000 V, or even much larger. Thebuffer layer is in an example a III-N buffer layer with a large bandgap.Therein III refers to Group III elements, now being Group 13 and Group 3elements, such as B, Al, Ga, In, Tl, Sc, Y and Lanthanide and Actinideseries. The buffer layer comprises a stack of layers, in an exampletypically the first one being a nucleation layer.

According to an optional aspect of the invention, the high electronmobility transistor further comprises an interface between the epitaxialIII-N semiconductor layer stack and the passivation stack, and whereinthe electron accepting dielectric layer provides an electron acceptorlevel at the interface.

This way, when the gate is not biased, the 2DEG of the high electronmobility transistor is depleted as the electrons of the 2DEG are flowingtowards the electron acceptor level at the interface between theepitaxial III-N semiconductor layer stack and the passivation stack. Inother words, the electron accepting dielectric layer provides a negativesurface charge via the presence of ionized electron accepting atoms atthe interface between the epitaxial III-N semiconductor layer stack andthe passivation stack, thereby depleting the electrons of the 2DEG ofthe high electron mobility transistor when the gate is not biased.

According to an optional aspect of the invention, the high electronmobility transistor further comprises an interface between the epitaxialIII-N semiconductor layer stack and the passivation stack, and whereinthe electron accepting dielectric layer provides an electron acceptorlevel in the passivation stack.

This way, when the gate is not biased, the 2DEG of the high electronmobility transistor is depleted as the electrons of the 2DEG are flowingtowards the electron acceptor level in the passivation stack. In otherwords, the electron accepting dielectric layer provides a negativesurface charge via the presence of ionized electron accepting atoms inthe passivation stack, thereby depleting the electrons of the 2DEG ofthe high electron mobility transistor when the gate is not biased.

According to an optional aspect of the invention, the electron acceptingdielectric layer comprises one or more of the following: MgSiN; MgAlN;MgSiAlN.

According to an optional aspect of the invention, the electron acceptingdielectric layer comprises one or more of the following:

-   -   Mg_(x)Si_(1-x)N, wherein x is comprised between 0.05 and 0.95;    -   Mg_(y)Al_(1-y)N, wherein y is comprised between 0.05 and 0.95;    -   Mg_(a)Si_(z)Al_(1-a-z)N, wherein a is comprised between 0.05 and        0.95 and wherein z is comprised between 0.05 and 0.95 and        wherein a+z is comprised between 0.1 and 1.

The term MgSiN relates to a composition comprising Mg, Si and N in anystochiometric ratio (Mg_(x)Si_(1-x)N) wherein x is comprised between0.05 and 0.95. The term MgAlN relates to a composition comprising Mg, Aland N in any stochiometric ratio (Mg_(y)Al_(1-y)N) wherein y iscomprised between 0.05 and 0.95. The term MgSiAlN relates to acomposition comprising Mg, Si, Al and N in any stochiometric ratio(Mg_(a)Si_(z)Al_(1-a-z)N) wherein a is comprised between 0.05 and 0.95and wherein z is comprised between 0.05 and 0.95 and wherein a+z iscomprised between 0.1 and 1.

According to an optional aspect of the invention, the electron acceptingdielectric layer comprises Mg_(x)Si_(1-x)N, wherein x is comprisedbetween 0.05 and 0.95.

This way, the electronic density in the channel of the high electronmobility transistor can be modulated by tuning the parameter x of thecomposition of the electron accepting dielectric layer.

According to an optional aspect of the invention, the electron acceptingdielectric layer comprises Mg_(y)Si_(1-y)N, wherein y is comprisedbetween 0.05 and 0.95.

This way, the electronic density in the channel of the high electronmobility transistor can be modulated by tuning the parameter y of thecomposition of the electron accepting dielectric layer.

According to an optional aspect of the invention, the electron acceptingdielectric layer comprises Mg_(a)Si_(z)Al_(1-a-z)N, wherein a iscomprised between 0.05 and 0.95 and wherein z in comprised between 0.05and 0.95 and wherein a+z is comprised between 0.1 and 1.

This way, the electronic density in the channel of the high electronmobility transistor can be modulated by tuning the parameter z of thecomposition of the electron accepting dielectric layer. The bandgap ofthe material of the electron accepting dielectric layer can be modulatedby tuning the parameters a and z.

According to an optional aspect of the invention, the electron acceptingdielectric layer is epitaxially grown on top of the epitaxial III-Nsemiconductor layer stack.

This way, the electron accepting dielectric layer is formed with theformation of the epitaxial III-N semiconductor layer stack. A fullycrystalline electron accepting dielectric layer is epitaxially grown ontop of the epitaxial III-N semiconductor layer stack. Alternatively, apartially crystalline electron accepting dielectric layer is epitaxiallygrown on top of the epitaxial III-N semiconductor layer stack. Theelectron accepting dielectric layer may be formed by ex-situ depositionwith the help of epitaxy tools like atomic layer deposition, alsoreferred to as ALD, chemical vapor deposition, also referred to as CVD,or physical vapor deposition, also referred to as PVD. Alternatively,the electron accepting dielectric layer may be formed by in-situdeposition in a MOCVD or a MBE chamber. Alternatively, the electronaccepting dielectric layer may be formed by depositing an amorphous filmof the same material and recrystallizing it using thermal anneal.

The difference in lattice constant between the first active III-N layerand the second active III-N layer produces a strain that can result indislocation of the active layer. This strain can result in interfacetrap states that slow the response of the device. The interface trapstates are associated with surface states created by dangling bonds,oxygen or hydroxyl adatoms, threading dislocations accessible at thesurface of the second active III-N layer. The electron acceptingdielectric layer epitaxially grown on top of the epitaxial III-Nsemiconductor layer stack therefore terminates and passivates danglingbonds on the surface of the second active III-N layer of the epitaxialIII-N semiconductor layer stack to limit the number of interfacialtraps, prevents oxygen or hydroxyl ions from migrating to and bonding onthe surface of the second active III-N layer, and helps to improve thedevice performance. In other words, the passivation stack reduces oreliminates the effects at the surface of the epitaxial III-Nsemiconductor layer which are responsible for degrading deviceperformance, such as for example drain current degradation, largerthreshold voltage fluctuation, larger off-current leakage, etc. due tothe presence of trapping states between the gate and the drain of thehigh electron mobility transistor. Additionally, the crystallinepassivation stack may have a lattice constant that matches with theepitaxial III-N semiconductor layer and the passivation stack on top ofit, hence providing suitable bond-matching to either side of theinterface and thus reducing the interfacial traps between the epitaxialIII-N semiconductor layer and the passivation stack. It also reduces theeffects of surface traps by providing a coherent termination of thesurface bonds. This way, a good interface can be realized by introducingthis crystalline passivation stack.

According to an optional aspect of the invention, the first active III-Nlayer comprises InAlGaN, and wherein the second active III-N layercomprises InAlGaN, and wherein the second active III-N layer comprises abandgap larger than a bandgap of the first active III-N layer andwherein the second active III-N layer comprises a polarization largerthan the polarization of the first active III-N layer.

This way, the use of different materials in adjacent first active III-Nlayer and second III-N layer causes polarization which contributes to aconductive 2DEG region near the junction between the first active III-Nlayer and the second active III-N layer, in particular in the firstactive III-N layer which comprises a bandgap narrower than the bandgapof the second active III-N layer.

The first active III-N layer for example has a thickness comprisedbetween 20 and 500 nm, preferably between 30 and 300 nm, more preferablybetween 50 and 250 nm, such as for example from 100 to 150 nm. Thesecond active III-N layer for example has a thickness comprised between10 to 100 nm, preferably between 20 to 50 nm. Such a combination ofthicknesses provides good characteristics for the active layer, forexample in terms of the 2DEG obtained.

The first active III-N layer comprises nitride and one or more of B, Al,Ga, In and Tl. The first active III-N layer for example comprises GaN.The second active III-N layer comprises nitride and one or more of B,Al, Ga, In, and Tl. The second active III-N layer for example comprisesAlGaN. The term AlGaN relates to a composition comprising Al, Ga and Nin any stochiometric ratio (Al_(x)Ga_(y)N) wherein x is comprisedbetween 0 and 1 and y is comprised between 0 and 1. Alternatively, thesecond active III-N layer for example comprises AlN. Alternatively, thesecond active III-N layer comprises InAlGaN. A composition such asInAlGaN comprises In in any suitable amount. Alternatively, both firstactive III-N layer and second active III-N layer comprise InAlGaN, andthe second active III-N layer comprises a bandgap larger than a bandgapof the first active III-N layer and wherein the second active III-Nlayer comprises a polarization larger than the polarization of the firstactive III-N layer. Alternatively, both first active III-N layer andsecond active III-N layer comprise BInAlGaN, and the second active III-Nlayer comprises a bandgap larger than a bandgap of the first activeIII-N layer and wherein the second active III-N layer comprises apolarization larger than the polarization of the first active III-Nlayer. Compositions of the active layer may be chosen in view ofcharacteristics to be obtained, and compositions may vary accordingly.For example, good results were obtained with a first active III-N layercomprising GaN of about 150 nm thickness and a second active III-N layercomprising AlGaN of about 20 nm thickness.

According to an optional aspect of the invention, the substratecomprises one or more of the following: Si, Silicon-On-Insulator,Silicon Carbide, Sapphire.

This way, the manufacturing of the high electron mobility of the presentinvention is compatible with existing manufacturing techniques developedfor the complementary metal-oxide-semiconductor technology andprocesses. In other words, the manufacturing of the high electronmobility transistor is CMOS-compatible as present features and presentprocess steps can be integrated therein without much additional effort.This reduces the complexity and the costs associated with manufacturingsuch as transistor. Preferably, the substrate is a Si substrate, such asa <111> Si substrate, and combinations of thereof, and substratescomprising initial layers, such as a stack of layers. Alternatively, thesubstrate of the high electron mobility transistor comprises Germanium,also referred to as Ge, or Ge-On-Insulator, etc. Alternatively, thesubstrate of the high electron mobility transistor comprises afree-standing GaN substrate, a free-standing AlN substrate.

According to an optional aspect of the invention, the passivation stackfurther comprises an oxide layer.

This way, the passivation stack of the high electron mobility transistorcomprises an oxide layer which acts as a gate insulator to the gate ofthe high electron mobility transistor. The oxide layer exhibits anelectrically clean interface to the gate, a high dielectric constant tomaximize electrostatic coupling between gate and 2DEG which results inan increase of the transconductance of the high electron mobilitytransistor and a sufficient thickness to avoid dielectric breakdown andleakage by quantum tunneling.

According to an optional aspect of the invention, the oxide layercomprises MgO.

This way, the gate insulator in the presence of the oxide layerdemonstrates a high dielectric constant, which allows for highercapacitance.

According to an optional aspect of the invention, the gate is formed ontop of the oxide layer.

This way, the oxide layer is formed to be comprised between the gate ofthe high electron mobility transistor and the electron acceptingdielectric layer. In other words, the electron accepting dielectriclayer is epitaxially formed on top of the epitaxial III-N semiconductorlayer, the oxide layer is formed on top of the electron acceptingdielectric layer and the gate is formed on top of the oxide layer.

According to an optional aspect of the invention:

-   -   the electron accepting dielectric layer comprises a passivation        surface in contact with the epitaxial III-N semiconductor layer        stack and a dielectric surface opposite to the passivation        surface; and    -   the second active III-N layer comprises a second passivation        surface in contact with the passivation surface of the electron        accepting dielectric layer, thereby defining a passivation        contact interface between the second active III-N layer and the        electron accepting dielectric layer.

According to an optional aspect of the invention:

-   -   the electron accepting dielectric layer comprises a passivation        surface in contact with the epitaxial III-N semiconductor layer        stack and a dielectric surface opposite to the passivation        surface;    -   the oxide layer comprises an oxide surface in contact with the        dielectric surface and a passivation insulating surface opposite        to the oxide surface;    -   the dielectric surface and the oxide surface extend such that        the oxide surface is in direct contact with the dielectric        surface along the full surface of the dielectric surface;    -   the gate comprises a biasing surface via which a voltage bias is        applied to the gate and a gate insulating surface opposite to        the biasing surface;    -   the gate is formed on top of the oxide layer, thereby defining        an insulating contact interface between the passivation        insulating surface and the gate insulating surface.

This way, an interface is formed between the electron acceptingdielectric layer comprising for example MgSiN or MgAlN or MgSiAlN andthe oxide layer comprising for example MgO. Additionally, an interfaceis formed between the oxide layer comprising for example MgO and thegate of the high electron mobility transistor. Alternatively, thedielectric surface and the oxide surface extend such that the oxidesurface is in direct contact with the dielectric surface along 10% to100% of the surface of the dielectric surface. The gate of the highelectron mobility transistor is biased via the gate insulating surface.In other words, a voltage is applied on the gate insulating surface tobias the high electron mobility transistor in operation.

According to an optional aspect of the invention, the insulating contactinterface extends such that the gate insulation surface is in directcontact with 10% to 100% of the passivation insulating surface.

This way, when the gate insulation surface extends along 100% of thepassivation insulating surface, the oxide layer is completely comprisedbetween the electron accepting dielectric layer and the gate, i.e. underthe gate of the high electron mobility transistor. When the gateinsulation surface is in direct contact with the passivation insulatingsurface but not along the full passivation insulating surface, the oxidelayer for example extends more than the gate and for example extendsbetween the source and the drain of the high electron mobilitytransistor.

According to an optional aspect of the invention, the electron acceptingdielectric layer comprises a passivation surface in contact with theepitaxial III-N semiconductor layer stack and a dielectric surfaceopposite to the passivation surface; and the second active III-N layercomprises a second passivation surface in contact with the passivationsurface of the electron accepting dielectric layer, thereby defining apassivation contact interface between the second active III-N layer andthe electron accepting dielectric layer.

According to an optional aspect of the invention, nitride atoms of theelectron accepting dielectric layer bond to Group III atoms of thesecond active III-N layer along the passivation contact interface.

This way, the potential barrier height in the second active III-N layerincreases and the 2DEG density of the high electron mobility transistordecreases when the gate is not biased, due to the presence of thepassivation stack and more particularly to the presence of the electronaccepting dielectric layer. Indeed, when the electron acceptingdielectric layer comprises for example MgSiN or MgAlN or MgSiAlN, the Mgatoms located at the passivation surface of the electron acceptingdielectric layer act as negatively ionized donors, which can partiallyneutralize the positive polarization charges of the second active III-Nlayer and thereby decrease the 2DEG density through polarizationeffects. In other words, the atoms of the electron accepting dielectriclayer, for example the Mg atoms when the electron accepting dielectriclayer comprises MgSiN or MgAlN or MgSiAlN, therefore serve as acceptorsfor electrons of the 2DEG, thereby depleting the channel when the gateof the high electron mobility transistor is not biased.

According to an optional aspect of the invention:

-   -   the passivation contact interface extends such that the        passivation surface is in direct contact with 10 to 30% of the        second passivation surface in a gate region; and    -   the passivation stack further comprises two electron donating        dielectric layers formed on top of the second active III-N layer        and on both sides of the electron accepting dielectric layer        such that each of two electron donating dielectric layers        comprises a III-N contact surface in direct contact with the        second active III-N layer.

This way, the electron accepting dielectric layer does not extend alongthe full surface of the second passivation surface of the second activeIII-N layer.

This way, the electron accepting dielectric layer is surrounded by anelectron donating dielectric layer on each side of the electronaccepting dielectric layer. In other words, the passivation surface ofthe electron accepting dielectric layer is in direct contact with thesecond passivation surface and on each side of the electron acceptingdielectric layer of the high electron mobility transistor is formed anelectron donating dielectric layer. This electron donating dielectriclayer improves the passivation of the second active III-N layer inregions in which no electron accepting dielectric layer is formed op topof the second active III-N layer. In other words, each of the electrondonating dielectric layers is in direct contact with the second activeIII-N layer.

According to an optional aspect of the invention, the electron donatingdielectric layers comprise SiN.

The electron donating dielectric layer is SiN with high density,deposited in-situ in an MOCVD reactor. The SiN may be stochiometric ornon-stochiometric. It has been shown experimentally by inventors thatfor example a HEMT structure that is capped with in-situ SiN is notaffected by processing steps, even those that have a high temperaturebudget. Alternatively, the electron donating dielectric layer comprisesAlSiN. The Al-doping allows increasing the bandgap of the dielectricmaterial. Alternatively, the electron donating dielectric layercomprises one or more of Si, Al, O and N. The electron donatingdielectric layer has a thickness of 1 to 500 nm, preferably 30 to 400nm, more preferably 50 to 300 nm, such as 100 to 200 nm. The in-situ SiNmay be thickened externally by PECVD or LPCVD SiN or SiO_(x), forexample for thicknesses beyond 500 nm, before any other processing takesplace. A thin electron donating dielectric layer allows the formation ofohmic contacts with a low resistance. Additionally, the electrondonating dielectric layer comprises Si which can diffuse in the AlGaNwhere it acts as a donor. The introduction of a donor type in the AlGaNlayer facilitates the ohmic contact formation reducing thereby thecontact resistance. The electron donating dielectric layer is formed ata temperature between 700° C. and 1300° C., between 700° C. and 1250°C., between 700° C. and 1100° C. It should be understood that when SiNis mentioned, a compound consisting of Si and N is meant. SiN caninclude Si₃N₄, but also other formulas are included, such as, but notlimited hereto, Si_(x)N_(y), being in different stochiometric ornon-stochiometric ratios. In the formula Si_(x)N_(y), x and y can bedefined as real numbers, with 0<x≤100 and 0<y≤100. When the epitaxialIII-N semiconductor layer stack is grown, NH₃ is kept flowing in thereaction chamber and the SiH₄ line is opened, allowing for growth of SiNand high temperature. After growth of SiN, the SiH₄ flow is stopped andthe structure is cooled down to room temperature while keeping the NH₃flow, to avoid desorption from the top layer.

According to an optional aspect of the invention, the electron donatingdielectric layers are epitaxially grown on top of the second activeIII-N layer.

It is an advantage that the crystallinity of in-situ grown SiN ismaintained by doping it or adding a species such as Al or B. When grownon top of the second active III-N layer, the in-situ SiN deforms toaccommodate to the strain resulting from the lattice mismatch betweenthe materials. It is well known that large lattice mismatch is a triggerto revert the epitaxial growth mode from a two-dimensional Franck-Vander Merwe layer-by-layer growth mode into a three-dimensionalVolker-Weber growth mode, which is then in turn more prone to turn intoan amorphous growth mode. A smaller atom than Si can thus beincorporated into the SiN, for example Al or B, to shrink the latticeconstant of the beta-phase SiN and match it better to the latticeconstant of the second active III-N layer. An additional advantage ofthe inclusion of Al in the SiN lattice is an improved resistance to dryetching in fluorine-based plasmas because of the interaction between Aland F which yields highly involatile AlF. The electron donatingdielectric layers are fully crystalline. Alternatively, the electrondonating dielectric layers are partially crystalline and comprise atleast a few crystalline monolayers at the interface with the secondpassivation surface of the second active III-N layer.

According to an optional aspect of the invention, the electron donatingdielectric layers each comprise a SiN III-N contact surface in directcontact with the second active III-N layer.

According to an optional aspect of the invention, the electron donatingdielectric layers are each etched away respectively in a source regionand a drain region.

This way, openings are defined in the electron donating dielectriclayers to uncover respectively a source region and a drain region inwhich the device terminals are to be formed. For example, aphotolithography step may be performed and the electron donatingdielectric layers may be etched away respectively in a source region andin a drain region. For example, the electron donating dielectric layerscan be removed by wet etching in HF or buffered HF or by dry etching ina RIE or ICP plasma tool in a fluorine chemistry.

Both dry and wet etches of the electron donating dielectric layers in afluorine chemistry will stop on the second active III-N layer which actsas an etch-stop with very high selectivity. For example, the etch of theelectron donating dielectric layers is done in a dry etching systembased on fluorine chemistry such as for example in an inductivelycoupled plasma system using SF₆ or CF₄ as etching gas and RF, or“platen”, and ICP, or “coil” etching powers of 10 W to 150 Wrespectively. This allows for thorough removal of the remaining electrondonating dielectric layer without removing the second active III-N layeror any of the layers below. Alternatively, the second active III-N layeris partially etched in a wet etch, for example in an alkaline solutionor in resist developer, thereby allowing to form respective ohmiccontacts in a source region and in a drain region partly in the activelayer.

According to an optional aspect of the invention, the passivationcontact interface extends such that the passivation surface is in directcontact with the second passivation surface along the full surface ofthe second passivation surface.

In this case, the electron accepting dielectric layer does extend alongthe full surface of the second passivation surface of the second activeIII-N layer.

According to an optional aspect of the invention, the electron acceptingdielectric layer is etched away respectively in a source region and adrain region.

In this case, the electron accepting dielectric layer fully covers thesecond active III-N layer, and no electron donating dielectric layersare formed on each side of the electron accepting dielectric layer.Openings are defined in the electron accepting dielectric layer touncover respectively a source region and a drain region in which thedevice terminals are to be formed. For example, a photolithography stepmay be performed and the electron accepting dielectric layer may beetched away respectively in a source region and in a drain region. Forexample, the electron accepting dielectric layer can be removed by dryetching.

According to an optional aspect of the invention, an ohmic contact isformed respectively in the source region and in the drain region.

The source and the drain contacts are ohmic contacts to the 2DEG and canbe made by depositing metal stacks, such as for example Ti/Al/Ni/Au,Ti/Al/Mo/Au, Ti/Al/Ti/Au, Ti/Al/Ti/W, Ti/Al/W, Ti/Al/W/Cr, Ta/Al/Ta,V/Al/Ni/Au, etc., in contact with the second active III-N layer of theactive layer. The second active III-N layer may be recessed prior tometal deposition. The contact properties may be further improved bythermal annealing, typically at a temperature comprised between 800° C.and 900° C., such as for example 850° C., in a nitrogen atmosphere or aforming gas atmosphere. Alternatively, additional metal interconnectlayers are defined using methods known to a person skilled in the art,to allow low resistivity current pathways for the gate, source and draincurrents.

According to an optional aspect of the invention, the epitaxial III-Nsemiconductor layer stack is adapted to host an electronic channelbetween the source region and the drain region when a positive biasvoltage is applied to the gate.

This way, once a bias voltage is applied to the gate which is largerthan the threshold voltage of the high electron mobility transistor,electrons flow in the electronic channel under the gate between thesource and the drain of the high electron mobility transistor.

According to an optional aspect of the invention, the electron acceptingdielectric layer is 0.1 to 3 nm thick.

This way, the electron accepting dielectric layer may be a single layer,such as for example a single MgSiN or MgAlN or MgSiAlN layer, andtherefore have the thickness of a single atomic monolayer.Alternatively, the electron accepting dielectric layer may comprise aplurality of MgSiN or MgAlN or MgSiAlN atomic layers, such as forexample two layers, three layers, four layers, five layers, ten layers,etc.

According to an optional aspect of the invention, the oxide layer is 1to 30 nm thick, preferably 3 nm to 10 nm thick.

According to an optional aspect of the invention, a thickness of the twoelectron donating dielectric layers is substantially equal to a combinedthickness of the electron accepting dielectric layer and the oxidelayer.

This way, the outer surface of the high electron mobility transistor isplanarized. Alternatively, the thickness of the two electron donatingdielectric layers is different from the combined thickness of theelectron accepting dielectric layer and of the oxide layer. In thiscase, a thick layer of SiN or of SiO_(x) could be deposited on thefinished high electron mobility transistor and could be planarized withfor example CMP, thereby obtaining a substantially flat surface.

According to an optional aspect of the invention, the recess in the gateregion extends completely through the second active III-N layer, therebyexposing the first active III-N layer.

According to an optional aspect of the invention, the electron acceptingdielectric layer extends in the recess such that the passivation surfaceis in direct contact with the first active III-N layer in the recess.

According to an optional aspect of the invention, the passivation stackfurther comprises an AlN layer, wherein said AlN layer comprises AlN andsuch that the AlN layer is in direct contact with the first active III-Nlayer in the recess; and wherein the electron accepting dielectric layerextends in the recess on top of the AlN layer.

This way, the device according to the present invention is a MOSFET witha fully recessed, or in other words removed barrier, wherein theelectron accepting dielectric layer is in contact with the channel layerin the first active III-N layer. This way, a true MOS type area underthe gate is formed. The electron accepting dielectric layer forms a goodinterface to the first active III-N layer with possibility to havecharge accumulation or inversion with positive bias, and the electronaccepting dielectric layer ensures there is no charge at negative biasin the channel. According to an optional aspect of the invention, theAlN layer comprises AlN and the thickness of the AlN layer is less than1 nm. Preferably, the AlN layer is a single monolayer of AlN. The AlNimproves the electronic mobility of the channel. Aluminum nitride, alsoreferred to as AlN, indeed demonstrates a wide bandgap, for example ofmore than 6 eV. Thanks to this wide bandgap of the AlN layer, theelectrons flowing feel less the surface roughness at the interfacebetween the first active III-N layer and the AlN layer, and theelectrons consequently have a better mobility.

According to a second aspect of the invention, there is provided amethod for manufacturing a high electron mobility transistor, the methodcomprising the steps of:

-   -   providing a substrate;    -   providing an epitaxial III-N semiconductor layer stack on top of        the substrate, wherein the providing the epitaxial III-N        semiconductor layer stack comprises providing an active layer        comprising:        -   a first active III-N layer; and        -   a second active III-N layer;    -   thereby forming a two dimensional Electron Gas between the first        active III-N layer and the second active III-N layer;    -   forming a recess in the second active III-N layer in a gate        region;    -   providing a passivation stack on top of the epitaxial III-N        semiconductor layer stack, wherein the passivation stack        comprises an electron accepting dielectric layer; and    -   providing a gate op top of the electron accepting dielectric        layer in the gate region such that the electron accepting        dielectric layer depletes the two dimensional Electron Gas when        the gate is not biased; wherein the electron accepting        dielectric layer extends in the recess and wherein the electron        accepting dielectric layer comprises magnesium nitride doped        with silicon and/or aluminum.

This way, the high electron mobility transistor of the present inventioncomprises an improved passivation stack which enhances the normally offoperation and thereby improves the enhancement mode of the high electronmobility transistor. Indeed, the passivation stack of the high electronmobility transistor according to the present invention comprises anelectron accepting dielectric layer which creates an acceptor level atthe interface between the passivation stack and the epitaxial III-Nsemiconductor layer stack. The electron accepting dielectric layer ofthe present invention depletes electrons from the two dimensionalElectron Gas, also referred to as 2DEG, when the gate of the highelectron mobility transistor is not biased. No channel is thereforepresent and no current flow occurs until the high electron mobilitytransistor is biased for operation. In particular, in operation, a biasvoltage is applied to the gate of the high electron mobility transistorin order to move the 2DEG from below the Fermi level. Once anothervoltage is applied between the source and the drain of the high electronmobility, the electrons in the 2DEG flow from source to drain. The highelectron mobility transistor according to the present invention istherefore suitable for applications such as e.g. power switching orintegrated logic for which negative polarity gate supply is undesired.The gate polarity of the high electron mobility transistor according tothe present invention is additionally desirable because of the addedsafety it provides.

Because of the recess in the second active III-N layer and in the gateregion, the high electron mobility transistor according to the presentinvention demonstrates a much higher leakage current from the gate tothe 2DEG than a similar high electron mobility transistor which does notcomprise a recess in the second active III-N layer. It is thereforenecessary to add a dielectric between the gate and the 2DEG. Dopingmagnesium nitride, or MgN, with silicon, also referred to as Si, and/orwith aluminum, also referred to as Al, increases the bandgap of theelectron accepting dielectric layer. When such electron acceptingdielectric layer extends in the recess in the second active III-N layerin the gate region, the leakage current is therefore reduced.Additionally, doping MgN with Si and/or Al increases the dielectricconstant of the electron accepting dielectric layer, thereby allowing abetter coupling between the gate and the 2DEG and demonstrating animproved conductance. The specific use of MgSiN or MgAlN or MgSiAlN inthe high electron mobility transistor according to the present inventionallows the device is be suitable for applications such as e.g. powerswitching or integrated logic for which negative polarity gate supply isundesired. The gate polarity of the high electron mobility transistoraccording to the present invention is additionally desirable because ofthe added safety it provides.

MgSiN depletes the channel of the high electron mobility transistor whenthe gate of the high electron mobility transistor is not biased andthereby improves the enhancement mode of the high electron mobilitytransistor. The material MgSiN of the electron accepting dielectriclayer exhibits a large bandgap, which makes it an interesting dielectriclayer for the gate dielectric of a high electron mobility transistor toprevent leakage. The scientific publication of Quirk J. B. et al.entitled “Band gap and electronic structure of MgSiN₂” published inApplied Physics Letters Volume 105, Issue 11 in September 2014 disclosesa bandgap of MgSiN equal to 6.3 eV. Aluminum doping, also referred to asAl doping increases the bandgap of the material of the electronaccepting dielectric layer, which results in an even more interestingdielectric layer for the passivation of a high electron mobilitytransistor and the use as gate dielectric as a higher bandgap will moreeffectively block electrons from leaking to or from the gate.Additionally, Al doping influences the etching rate in Fluor-basedplasma of the passivation stack. In other words, doping the passivationstack with Aluminum creates an etch stop layer for Fluor-based plasmaetching. The bandgap of MgSiAlN is expected to be higher than 6 eV.

The electron accepting dielectric layer may be selectively grown on topof the active layer of the high electron mobility transistor. In thiscase, a dielectric layer, for example SiN or SiO_(x), is deposited atthe surface of the second active III-N layer. This dielectric layer mayfurther comprise a sacrificial SiO_(x) layer on top of the SiN when thedielectric layer comprises SiN. The dielectric layer and the sacrificiallayer when present are then patterned by for example a lithography stepand are then removed in the gate region of the high electron mobilitytransistor. Then the electron accepting dielectric layer is selectivelygrown by MOCVD or by MBE in the gate region. Alternatively, the electronaccepting dielectric layer is grown via blanket growth on top of theactive layer by MOCVD or be MBE. In this case, the electron acceptingdielectric layer is then removed except in a gate region of the highelectron mobility transistor, for example the electron acceptingdielectric layer is etched away except in a gate region of the highelectron mobility transistor.

According to an optional aspect of the invention, providing the electronaccepting dielectric layer corresponds to epitaxially growing theelectron accepting dielectric layer.

According to an optional aspect of the invention, providing thepassivation stack on top of said epitaxial III-N semiconductor layerstack corresponds to epitaxially growing the passivation stack on top ofthe epitaxial III-N semiconductor layer stack.

According to an optional aspect of the invention, epitaxially growing isby MOCVD or MBE.

The crystalline electron accepting dielectric layer can be formedin-situ by epitaxial growth in a MOCVD or a MBE chamber. The firstactive III-N layer and the second active III-N layer can be formedin-situ by epitaxial growth in a MOCVD or a MBE chamber.

According to an optional aspect of the invention, the method furthercomprises the steps of:

-   -   etching the passivation stack in a source region and a drain        region; and    -   forming an ohmic contact respectively in the source region and        in the drain region.

According to an optional aspect of the invention, the method furthercomprises the steps of:

-   -   providing an electron donating dielectric layer on top of the        electron accepting dielectric layer;    -   locally removing the electron donating dielectric layers in a        gate region, thereby forming an opening in the electron donating        dielectric layer and locally exposing the electron accepting        dielectric layer;    -   forming a gate electrode in the gate region;    -   locally removing the electronic donating dielectric layer in a        source region and in a drain region, thereby respectively        forming an opening in the electron donating dielectric layer in        the source region and locally exposing the second active III-N        layer in the source region, and forming an opening in the        electron donating dielectric layer in the drain region and        locally exposing the second active III-N layer in the drain        region; and    -   forming an ohmic contact in the source region and an ohmic        contact in the drain region.

This way, a gate electrode is provided in the gate region of the highelectron mobility transistor is formed. Forming a gate electrode in thegate region comprise plurality of process steps. For example this stepcomprises depositing photoresist and performing a lithography stepdefining the foot of the gate contact by for example partially removingthe oxide layer. In this way, some layers of the oxide layer remainbelow the gate of the high electron mobility transistor and form a gatedielectric to reduce trapping effects and leakage current. The gateelectrode is for example a Metal-Oxide-Semiconductor gate, also referredto MOS gate, and can be made by depositing metal stacks, such as forexample comprising Ni, Pt, W, WN, or TiN and capped by Al, Au or Cu.Metal patterns are consecutively defined by performing lift-off of themetal on top of the photoresist. Alternatively, the gate metal stack isdeposited, for example comprising Ni, Pt, W, WN, or TiN and capped byAl, Au or Cu. Then the photoresist and the lithography steps areperformed, and the thus defined photoresist patterns act as a mask forthe dry etching of the metal stacks in areas where it is unwanted. Nextthe photoresist is removed.

Forming an ohmic contact in the source region and forming an ohmiccontact in the drain region comprise plurality of process steps. Forexample, this is done by starting with depositing photoresist anddefining the respective areas of the respective ohmic contacts with alithography step. The electron donating dielectric layers are thenpartially or fully removed respectively in a source region and in adrain region. Alternatively, the electron accepting dielectric layer isfully removed in a source region and in a drain region. Once the areasof the ohmic contacts are defined, i.e. when the source region and thedrain region have been defined, a metal layer or a stack of metal layerscan be deposited, for example by thermal evaporation, or by sputtering,or by e-beam evaporation. Metal patterns are consecutively defined byperforming lift-off of the metal, on top of the photoresist and not incontact with the second active III-N layer. Alternatively, thephotoresist is first removed and the metal stack comprising for exampleTi and Al is deposited and then a second photoresist deposition andphotolithography steps are performed to allow dry etching of the metalstack in areas where it is unwanted and removing the photoresist. Thedefined ohmic contacts may then be subjected to one or more alloyingsteps, for example a rapid thermal annealing step for a duration of oneminute in a reduced or inert atmosphere such as for example hydrogen orforming gas or nitrogen gas at a temperature for example between 800° C.and 900° C.

According to a third aspect of the invention, there is provided a use ofan electron accepting dielectric layer comprising magnesium nitridedoped with silicon and/or aluminum in a high electron mobilitytransistor for analog applications, the high electron mobilitytransistor comprising:

-   -   a substrate;    -   an epitaxial III-N semiconductor layer stack on top of the        substrate, the epitaxial III-N semiconductor layer stack        comprising an active layer, the active layer comprising:        -   a first active III-N layer; and        -   a second active III-N layer comprising a recess in a gate            region; with a two dimensional Electron Gas between the            first active III-N layer and the second active III-N layer;    -   a gate on top of the epitaxial III-N semiconductor layer stack        and in the gate region; and    -   a passivation stack between the epitaxial III-N semiconductor        layer stack and the gate, wherein the passivation stack        comprises an electron accepting dielectric layer, and wherein        the electron accepting dielectric layer extends in the recess        and wherein the electron accepting dielectric layer comprises        magnesium nitride doped with silicon and/or aluminum; for        depletion of the two dimensional Electron Gas when the gate is        not biased.

Nitride atoms of the electron accepting dielectric layer bond to GroupIII atoms of the second active III-N layer along the passivation contactinterface. Incorporating Mg atoms in the passivation stack of a highelectron mobility transistor therefore creates an electron acceptinglevel at the interface between the epitaxial III-N semiconductor layerstack and the passivation stack, thereby depleting electrons from the2DEG channel of the high electron mobility transistor when the gate isnot biased. A negative surface charge is provided by the ionized Mgacceptors at the interface between the epitaxial III-N semiconductorlayer stack and the passivation stack. The specific use of MgSiN orMgAlN or MgSiAlN in the electron accepting dielectric layer enhances thenormally off operation and thereby improves the enhancement mode of thehigh electron mobility transistor. Indeed, the electron acceptingdielectric layer creates an acceptor level at the interface between thepassivation stack and the epitaxial III-N semiconductor layer stack. Theelectron accepting dielectric layer of the present invention depleteselectrons from the two dimensional Electron Gas, also referred to as2DEG, when the gate of the high electron mobility transistor is notbiased. No channel is therefore present and no current flow occurs untilthe high electron mobility transistor is biased for operation. Inparticular, in operation, a bias voltage is applied to the gate of thehigh electron mobility transistor in order to move the 2DEG from belowthe Fermi level. Once another voltage is applied between the source andthe drain of the high electron mobility, the electrons in the 2DEG flowfrom source to drain.

Because of the recess in the second active III-N layer and in the gateregion, the high electron mobility transistor according to the presentinvention demonstrates a much higher leakage current from the gate tothe 2DEG than a similar high electron mobility transistor which does notcomprise a recess in the second active III-N layer. It is thereforenecessary to add a dielectric between the gate and the 2DEG. Dopingmagnesium nitride, or MgN, with silicon, also referred to as Si, and/orwith aluminum, also referred to as Al, increases the bandgap of theelectron accepting dielectric layer. When such electron acceptingdielectric layer extends in the recess in the second active III-N layerin the gate region, the leakage current is therefore reduced.Additionally, doping MgN with Si and/or Al increases the dielectricconstant of the electron accepting dielectric layer, thereby allowing abetter coupling between the gate and the 2DEG and demonstrating animproved conductance. The specific use of MgSiN or MgAlN or MgSiAlN inthe high electron mobility transistor according to the present inventionallows the device is be suitable for applications such as e.g. powerswitching or integrated logic for which negative polarity gate supply isundesired. The gate polarity of the high electron mobility transistoraccording to the present invention is additionally desirable because ofthe added safety it provides.

MgSiN depletes the channel of the high electron mobility transistor whenthe gate of the high electron mobility transistor is not biased andthereby improves the enhancement mode of the high electron mobilitytransistor. The material MgSiN of the electron accepting dielectriclayer exhibits a large bandgap, which makes it an interesting dielectriclayer for the gate dielectric of a high electron mobility transistor toprevent leakage. The scientific publication of Quirk J. B. et al.entitled “Band gap and electronic structure of MgSiN₂” published inApplied Physics Letters Volume 105, Issue 11 in September 2014 disclosesa bandgap of MgSiN equal to 6.3 eV. Aluminum doping, also referred to asAl doping increases the bandgap of the material of the electronaccepting dielectric layer, which results in an even more interestingdielectric layer for the passivation of a high electron mobilitytransistor and the use as gate dielectric as a higher bandgap will moreeffectively block electrons from leaking to or from the gate.Additionally, Al doping influences the etching rate in Fluor-basedplasma of the passivation stack. In other words, doping the passivationstack with Aluminum creates an etch stop layer for Fluor-based plasmaetching. The bandgap of MgAlSiN is expected to be higher than 6 eV.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C schematically illustrate the charge distribution in aprior art high electron mobility transistor (FIG. 1A), in a prior arthigh electron mobility transistor comprising a recess in a second activeIII-N layer (FIG. 1B) and in a high electron mobility transistoraccording to the present invention (FIG. 1C).

FIGS. 2A to 2C schematically illustrate an embodiment of a semiconductorstructure according to the present invention, wherein said electronaccepting dielectric layer is deposited in a gate region after etching apassivation stack in said gate region.

FIGS. 3A and 3B schematically illustrate an embodiment of a highelectron mobility transistor according to the present invention, whereina passivation stack extends fully on top of an epitaxial III-Nsemiconductor layer stack.

FIGS. 4A to 4C schematically illustrate an embodiment of a high electronmobility transistor according to the present invention, wherein anelectron accepting dielectric layer is deposited on top of a partialrecess in FIG. 4B or a full recess in FIG. 4C formed in a second activeIII-N layer of an epitaxial III-N semiconductor layer stack.

FIG. 5A to 5C schematically illustrate an embodiment of a semiconductorstructure according to the present invention, wherein said semiconductorstructure comprises a partial recess formed in a second active III-Nlayer.

FIGS. 6A to 6C schematically illustrate an embodiment of a semiconductorstructure according to the present invention, wherein said electronaccepting dielectric layer is deposited in a gate region after etching apassivation stack in said gate region, and wherein said semiconductorstructure comprises a recess formed in a second active III-N layer.

FIGS. 7A to 7C schematically illustrate an embodiment of a high electronmobility transistor according to the present invention.

FIG. 8A to 8C schematically illustrate an embodiment of a semiconductorstructure according to the present invention, wherein said semiconductorstructure comprises a full recess formed in a second active III-N layer.

FIG. 9A to 9C schematically illustrate an embodiment of a high electronmobility transistor according to the present invention, wherein saidhigh electron mobility transistor comprises a full recess formed in asecond active III-N layer.

FIG. 10A to 10C schematically illustrate an embodiment of asemiconductor structure according to the present invention, wherein saidsemiconductor structure comprises a full recess formed in a secondactive III-N layer and further comprises an AlN layer.

FIG. 11A to 11C schematically illustrate an embodiment of a highelectron mobility transistor according to the present invention, whereinsaid high electron mobility transistor comprises a full recess formed ina second active III-N layer and further comprises an AlN layer.

FIG. 12 schematically illustrates an embodiment of the steps of a methodaccording to the present invention.

DETAILED DESCRIPTION OF EMBODIMENT(S)

According to a prior art embodiment shown in FIG. 1A, the chargedistribution in a standard high electron mobility transistor comprisinga 2DEG 21 is schematically illustrated. In this case, the barrier 201 islarge, and ionized surface donors 203 are present above the Fermi level200 while non-ionized surface donors 204 are present below the Fermilevel 200. According to a prior art embodiment shown in FIG. 1B, thecharge distribution in a standard high electron mobility transistorcomprising a 2DEG 21 and in which a recess is formed in the secondactive III-N layer is schematically illustrated. In this case, thebarrier 201 is narrower than the barrier 201 of FIG. 1A, and as aresult, the leakage current in this structure is much higher than in theHEMT 1 of FIG. 1A. Ionized surface donors 203 are present above theFermi level 200 while non-ionized surface donors 204 are present belowthe Fermi level 200 in FIG. 1B. According to an embodiment shown in FIG.1C, the charge distribution in a high electron mobility transistoraccording to the present invention comprising a 2DEG 21 and in which arecess is formed in the second active III-N layer and comprising anelectron accepting dielectric layer comprising MgSiN or MgAlN or MgSiAlNis schematically illustrated. In this case, the barrier 201 is narrowerthan the barrier 201 of FIG. 1A, and as a result, the leakage current inthis structure is much higher than in the HEMT 1 of FIG. 1A. But at thesame time, the electron accepting dielectric layer comprises MgSiN orMgAlN or MgSiAlN which demonstrates a large bandgap and which,positioned between the gate and the channel of the HEMT, thereforereduces the leakage current. Ionized surface donors 203 are presentabove the Fermi level 200 while non-ionized surface donors 204 arepresent below the Fermi level 200 in FIG. 1C. The charge exchangebetween the barrier surface donor level and the electron acceptingdielectric level created by the electron accepting dielectric layerleads to the modification of surface potential different from the Fermilevel 200. In FIG. 1C, the number of ionized surface donors 203 ishigher than the number of ionized surface donors 203 in HEMT which doesnot comprise a recess in the second active III-N layer nor an electronaccepting dielectric layer, and it is also higher than the number ofionized surface donors 203 in HEMT which does comprise a recess in thesecond active III-N layer but which does not comprise an electronaccepting dielectric layer. The combination of the presence of therecess in the second active III-N layer and of the specific choice ofMgSiN or MgAlN or MgSiAlN as material for the electron acceptingdielectric layer improves the depletion of the electrons from the 2DEGwhen the gate of the HEMT is not biased and therefore enhances thenormally off operation of the HEMT, thereby improving its enhancementmode. A high electron mobility transistor of the present inventioncomprises an improved passivation stack which enhances the normally offoperation and thereby improves the enhancement mode of the high electronmobility transistor. Indeed, the passivation stack of the high electronmobility transistor according to the present invention comprises anelectron accepting dielectric layer which creates an acceptor level atthe interface between the passivation stack and the epitaxial III-Nsemiconductor layer stack. The electron accepting dielectric layer ofthe present invention depletes electrons from the two dimensionalElectron Gas, also referred to as 2DEG, when the gate of the highelectron mobility transistor is not biased. No channel is thereforepresent and no current flow occurs until the high electron mobilitytransistor is biased for operation. In particular, in operation, a biasvoltage is applied to the gate of the high electron mobility transistorin order to move the 2DEG from below the Fermi level 200. Once anothervoltage is applied between the source and the drain of the high electronmobility, the electrons in the 2DEG flow from source to drain. The highelectron mobility transistor according to the present invention istherefore suitable for applications such as e.g. power switching orintegrated logic for which negative polarity gate supply is undesired.The gate polarity of the high electron mobility transistor according tothe present invention is additionally desirable because of the addedsafety it provides. Because of the recess in the second active III-Nlayer and in the gate region, the high electron mobility transistoraccording to the present invention demonstrates a much higher leakagecurrent from the gate to the 2DEG than a similar high electron mobilitytransistor which does not comprise a recess in the second active III-Nlayer. Doping magnesium nitride, or MgN, with silicon, also referred toas Si, and/or with aluminum, also referred to as Al, increases thebandgap of the electron accepting dielectric layer. When such electronaccepting dielectric layer extends in the recess in the second activeIII-N layer in the gate region, and between the gate and the 2DEG, theleakage current is therefore reduced. Additionally, doping MgN with Siand/or Al increases the dielectric constant of the electron acceptingdielectric layer, thereby allowing a better coupling between the gateand the 2DEG and demonstrating an improved conductance. Additionally,thanks to the recess in the second active III-N layer in the gateregion, the electron accepting dielectric layer is brought closer to the2DEG, thereby improving the effect of depletion of electrons from the2DEG by the electron accepting dielectric layer when the gate is notbiased. Nitride atoms of the electron accepting dielectric layer bond toGroup III atoms of the second active III-N layer along the passivationcontact interface. Incorporating Mg atoms in the passivation stack of ahigh electron mobility transistor therefore creates an electronaccepting level at the interface between the epitaxial III-Nsemiconductor layer stack and the passivation stack, thereby depletingelectrons from the 2DEG channel of the high electron mobility transistorwhen the gate is not biased. A negative surface charge is provided bythe ionized Mg acceptors at the interface between the epitaxial III-Nsemiconductor layer stack and the passivation stack. MgSiN depletes thechannel of the high electron mobility transistor when the gate of thehigh electron mobility transistor is not biased and thereby improves theenhancement mode of the high electron mobility transistor. The materialMgSiN of the electron accepting dielectric layer exhibits a large orwide bandgap larger than 6.3 eV, which makes it an interestingdielectric layer for the gate dielectric of a high electron mobilitytransistor to prevent leakage. Aluminum doping, also referred to as Aldoping increases the bandgap of the material of the electron acceptingdielectric layer, which results in an even more interesting dielectriclayer for the passivation of a high electron mobility transistor and theuse as gate dielectric as a higher bandgap will more effectively blockelectrons from leaking to or from the gate. Additionally, Al dopinginfluences the etching rate in Fluor-based plasma of the passivationstack. In other words, doping the passivation stack with aluminumcreates an etch stop layer for Fluor-based plasma etching. The bandgapof MgAlSiN is expected to be higher than 6 eV.

According to an embodiment shown in FIGS. 2A to 2C, a high electronmobility transistor according to the present invention is manufacturedas schematically illustrated by the different steps of FIGS. 2A to 2C.On FIG. 2A, a semiconductor structure according to the present inventioncomprises a substrate 10 and an epitaxial III-N semiconductor layerstack 20. The epitaxial III-N semiconductor layer stack 20 comprises afirst active III-N layer 22 and a second active III-N layer 23, with atwo dimensional Electron Gas 21 between the first active III-N layer 22and the second active III-N layer 23. The passivation stack 40 is thenetched away in a gate region 31 and the second active III-N layer 23 ispartially etched in a gate region 31 using the passivation stack 40 as amask. In other words, a recess 24 is formed in the second active III-Nlayer 23 in the gate region 31. This can be achieved by etching in aplasma etching tool such as Reactive Ion Etching or RIE or preferably inan Inductively Coupled Plasma or ICP tool. The reagent gases can be Cl₂or BCl₃. Alternatively, a digital etching process can be used, whereasconsecutively and iteratively, first the top surface of the secondactive III-N layer is oxidized for example in O₂, O₃ or N₂O plasma,after which the formed oxide is etched away e.g. in SF₆ or CF₄ plasma.The first active III-N layer 22 for example has a thickness comprisedbetween 20 and 500 nm, preferably between 30 and 300 nm, more preferablybetween 50 and 250 nm, such as for example from 100 to 150 nm. Thesecond active III-N layer 23 for example has a thickness comprisedbetween 10 to 100 nm, preferably between 20 to 50 nm. Such a combinationof thicknesses provides good characteristics for the active layer, forexample in terms of the 2DEG 21 obtained. The first active III-N layer22 comprises nitride and one or more of B, Al, Ga, In and Tl. The firstactive III-N layer 22 for example comprises GaN. The second active III-Nlayer 23 comprises nitride and one or more of B, Al, Ga, In, and Tl. Thesecond active III-N layer 23 for example comprises AlGaN. The term AlGaNrelates to a composition comprising Al, Ga and N in any stochiometricratio (Al_(x)Ga_(y)N) wherein x is comprised between 0 and 1 and y iscomprised between 0 and 1. Alternatively, the second active III-N layer23 for example comprises AlN. Alternatively, the second active III-Nlayer 23 comprises InAlGaN. A composition such as InAlGaN comprises Inin any suitable amount. Alternatively, both first active III-N layer 22and second active III-N layer 23 comprise InAlGaN, and the second activeIII-N layer 23 comprises a bandgap larger than a bandgap of the firstactive III-N layer 22 and wherein the second active III-N layer 23comprises a polarization larger than the polarization of the firstactive III-N layer 22. Alternatively, both first active III-N layer 22and second active III-N layer 23 comprise BInAlGaN, and the secondactive III-N layer 23 comprises a bandgap larger than a bandgap of thefirst active III-N layer 22 and wherein the second active III-N layer 23comprises a polarization larger than the polarization of the firstactive III-N layer 22. Compositions of the active layer may be chosen inview of characteristics to be obtained, and compositions may varyaccordingly. For example, good results were obtained with a first activeIII-N layer 22 comprising GaN of about 150 nm thickness and a secondactive III-N layer 23 comprising AlGaN of about 20 nm thickness. Asvisible on FIG. 2B, a passivation stack 40 is formed on top of theepitaxial semiconductor layer stack 20. The passivation stack 40comprises an electron donating dielectric layer. The passivation stack40 for example comprises SiN. The passivation stack 40 comprises SiNwith high density, deposited in-situ in an MOCVD reactor. The SiN may bestochiometric or non-stochiometric. The in-situ SiN may be thickenedexternally by PECVD or LPCVD SiN or SiO_(x), for example for thicknessesbeyond 500 nm, before any other processing takes place. According to analternative embodiment, the passivation stack 40 comprises SiO₂.According to a further alternative embodiment, the passivation stack 40of FIG. 2B comprises AlSiN. The Al-doping allows increasing the bandgapof the dielectric material. Alternatively, the passivation stack of FIG.2B comprise one or more of Si, Al, O and N. The passivation stack 40 isthen etched away in a gate region 31, thereby exposing the secondpassivation surface 230 of the second active III-N layer 23, and therebyforming two electron donating dielectric layers 43; 44 on top of thesecond active III-N layer 23 and on both sides of the gate region 31,such that each of the two electron donating dielectric layers 43; 44comprises a III-N contact surface 430; 440 in direct contact with thesecond active III-N layer 23. According to an alternative embodiment,the second active III-N layer 23 is partially etched in a gate region31. As visible in FIG. 2C, an electron accepting dielectric layer 41 isthen formed in the gate region 31 and extends in the recess 24 of thesecond active III-N layer 23. The electron accepting dielectric layer 41comprises a passivation surface 410 in contact with the second activeIII-N layer 23 of the epitaxial III-N semiconductor layer stack 20. Theelectron accepting dielectric layer 41 further comprises a dielectricsurface 411 opposite to the passivation surface 410. The second activeIII-N layer 23 comprises a second passivation surface 230 in contactwith the passivation surface 410 of the electron accepting dielectriclayer 41, thereby defining a passivation contact interface 231 betweenthe second active III-N layer 23 and the electron accepting dielectriclayer 41. The passivation contact interface 231 extends such that thepassivation surface 410 is in direct contact with 10 to 30% of thesecond passivation surface in a gate region 31. The electron acceptingdielectric layer 41 comprises for example Mg_(x)Si_(1-x)N, wherein x iscomprised between 0.05 and 0.95. According to an alternative embodiment,the electron accepting dielectric layer 41 comprises Mg_(y)Al_(1-y)N,wherein y is comprised between 0.05 and 0.95. According to a furtheralternative embodiment, the electron accepting dielectric layercomprises Mg_(a)Si_(z)Al_(1-a-z)N, wherein a is comprised between 0.05and 0.95 and wherein z is comprised between 0.05 and 0.95 and the a+z iscomprised between 0.1 and 1. The MgSiN or the MgAlN, or the MgSiAlN areepitaxially grown on top of the epitaxial III-N semiconductor layerstack 20, preferably on top of the second active III-N layer 23.

According to an embodiment shown in FIGS. 3A and 3B, a high electronmobility transistor according to the present invention is manufacturedas schematically illustrated by the different steps of FIGS. 2A to 2C.Components having identical reference numbers to components in FIG. 2Ato 2C perform the same function. As visible in FIG. 3A, a semiconductorstructure 1 according to the present invention comprises a substrate 10and an epitaxial III-N semiconductor layer stack 20. The epitaxial III-Nsemiconductor layer stack 20 comprises a first active III-N layer 22 anda second active III-N layer 23, with a two dimensional Electron Gas 21between the first active III-N layer 22 and the second active III-Nlayer 23. The second active III-N layer 23 is partially etched in a gateregion 31. In other words, a recess 24 is formed in the second activeIII-N layer 23 in the gate region 31. This can be achieved by etching ina plasma etching tool such as Reactive Ion Etching or RIE or preferablyin an Inductively Coupled Plasma or ICP tool. The reagent gases can beCl₂ or BCl₃. Alternatively, a digital etching process can be used,whereas consecutively and iteratively, first the top surface of thesecond active III-N layer is oxidized for example in O₂, O₃ or N₂Oplasma, after which the formed oxide is etched away e.g. in SF₆ or CF₄plasma. The first active III-N layer 22 for example has a thicknesscomprised between 20 and 500 nm, preferably between 30 and 300 nm, morepreferably between 50 and 250 nm, such as for example from 100 to 150nm. The second active III-N layer 23 for example has a thicknesscomprised between 10 to 100 nm, preferably between 20 to 50 nm. Such acombination of thicknesses provides good characteristics for the activelayer, for example in terms of the 2DEG 21 obtained. The first activeIII-N layer 22 comprises nitride and one or more of B, Al, Ga, In andTl. The first active III-N layer 22 for example comprises GaN. Thesecond active III-N layer 23 comprises nitride and one or more of B, Al,Ga, In, and Tl. The second active III-N layer 23 for example comprisesAlGaN. The term AlGaN relates to a composition comprising Al, Ga and Nin any stochiometric ratio (Al_(x)Ga_(y)N) wherein x is comprisedbetween 0 and 1 and y is comprised between 0 and 1. Alternatively, thesecond active III-N layer 23 for example comprises AlN. Alternatively,the second active III-N layer 23 comprises InAlGaN. A composition suchas InAlGaN comprises In in any suitable amount. Alternatively, bothfirst active III-N layer 22 and second active III-N layer 23 compriseInAlGaN, and the second active III-N layer 23 comprises a bandgap largerthan a bandgap of the first active III-N layer 22 and wherein the secondactive III-N layer 23 comprises a polarization larger than thepolarization of the first active III-N layer 22. Alternatively, bothfirst active III-N layer 22 and second active III-N layer 23 compriseBInAlGaN, and the second active III-N layer 23 comprises a bandgaplarger than a bandgap of the first active III-N layer 22 and wherein thesecond active III-N layer 23 comprises a polarization larger than thepolarization of the first active III-N layer 22. Compositions of theactive layer may be chosen in view of characteristics to be obtained,and compositions may vary accordingly. For example, good results wereobtained with a first active III-N layer 22 comprising GaN of about 150nm thickness and a second active III-N layer 23 comprising AlGaN ofabout 20 nm thickness. A passivation stack 40 is formed in FIG. 3A ontop of the second active III-N layer 23 by forming an electron acceptingdielectric layer 41 and an oxide layer 42. The electron acceptingdielectric layer 41 extends in the recess 24 of the second active III-Nlayer 23 The electron accepting dielectric layer 41 comprises apassivation surface 410 in contact with the second active III-N layer 23of the epitaxial III-N semiconductor layer stack 20. The electronaccepting dielectric layer 41 further comprises a dielectric surface 411opposite to the passivation surface 410. The second active III-N layer23 comprises a second passivation surface 230 in contact with thepassivation surface 410 of the electron accepting dielectric layer 41,thereby defining a passivation contact interface 231 between the secondactive III-N layer 23 and the electron accepting dielectric layer 41.The passivation contact interface 231 extends such that the passivationsurface 410 is in direct contact with the second passivation surface 230along the full surface of the second passivation surface 230. Theelectron accepting dielectric layer 41 comprises for exampleMg_(x)Si_(1-x)N, wherein x is comprised between 0.05 and 0.95. Accordingto an alternative embodiment, the electron accepting dielectric layer 41comprises Mg_(y)Al_(1-y)N, wherein y is comprised between 0.05 and 0.95.According to a further alternative embodiment, the electron acceptingdielectric layer comprises Mg_(a)Si_(z)Al_(1-a-z)N, wherein a iscomprised between 0.05 and 0.95 and wherein z is comprised between 0.05and 0.95 and the a+z is comprised between 0.1 and 1. The MgSiN or theMgAlN, or the MgSiAlN are epitaxially grown on top of the epitaxialIII-N semiconductor layer stack 20, preferably on top of the secondactive III-N layer 23. As shown in FIG. 3A, the passivation stack 40further comprises an oxide layer 42. The passivation stack 40 and moreparticularly the electron accepting dielectric layer 41 and the oxidelayer 42 are for example grown by MOCVD. According to an alternativeembodiment, the passivation stack 40 is grown by MBE. The oxide layer 42for example comprises MgO. According to an alternative embodiment, theoxide layer 42 comprises AlO_(x) or SiO_(x), or alloys thereof.According to a further alternative embodiment, the oxide layer 42comprises a gate dielectric such as for example HfO_(x), ZrO_(x), etc.The oxide layer 42 comprises an oxide surface 420 in contact with thedielectric surface 411 and a passivation insulating surface 421 oppositeto the oxide surface 420. The dielectric surface 411 and the oxidesurface 420 extend such that the oxide surface 420 is in direct contactwith the dielectric surface 411 along the full surface of the dielectricsurface 411. On FIG. 3A, a gate 30 is formed on top of the passivationstack 40 in the gate region 31. The gate 30 comprises a biasing surface300 via which a voltage bias is applied to the gate 30 and a gateinsulating surface 301 opposite to the biasing surface 300. Moreparticularly, a gate is formed in the gate region 31 on top of the oxidelayer 42, thereby defining an insulating contact interface 423 betweenthe passivation insulating surface 421 and the gate insulating surface301. The insulating contact interface 423 extends such that the gateinsulating surface 301 is in direct contact with 10 to 30% of thepassivation insulating surface 421. As visible in FIG. 3B, thepassivation stack 40 is etched away in a source access region and adrain access region. In other words, the electron accepting dielectriclayer 41 and the oxide layer 42 are etched away in a source accessregion and in a drain access region, thereby exposing the second activeIII-N layer 23 in a source region 51 and a drain region 52. According toan alternative embodiment, the second active III-N layer 23 is partiallyetched in a wet etch, for example in an alkaline solution or in resistdeveloper, thereby allowing to form respective ohmic contacts in asource region 51 and in a drain region 52 partly in the second activeIII-N layer 23. Once the areas of the ohmic contacts are defined, i.e.when the source region 51 and the drain region 52 have been defined, ametal layer or a stack of metal layers can be deposited, for example bythermal evaporation, or by sputtering, or by e-beam evaporation. Metalpatterns are consecutively defined by performing lift-off of the metal,on top of the photoresist and not in contact with the second activeIII-N layer 23. Alternatively, the photoresist is first removed and themetal stack comprising for example Ti and Al is deposited and then asecond photoresist deposition and photolithography steps are performedto allow dry etching of the metal stack in areas where it is unwantedand removing the photoresist. The defined ohmic contacts may then besubjected to one or more alloying steps, for example a rapid thermalannealing step for a duration of one minute in a reduced or inertatmosphere such as for example hydrogen or forming gas or nitrogen gasat a temperature for example between 800° C. and 900° C. A high electronmobility transistor 1 according to the present invention is obtained.

According to an embodiment shown in FIGS. 4A and 4B, a high electronmobility transistor according to the present invention is manufacturedas schematically illustrated by the different steps of FIGS. 4A and 4B.Components having identical reference numbers to components in FIG. 2Ato 2C and FIGS. 3A and 3B perform the same function. As visible in FIG.4A, a semiconductor structure 1 according to the present inventioncomprises a substrate 10 and an epitaxial III-N semiconductor layerstack 20. The epitaxial III-N semiconductor layer stack 20 comprises afirst active III-N layer 22 and a second active III-N layer 23, with atwo dimensional Electron Gas 21 between the first active III-N layer 22and the second active III-N layer 23. The first active III-N layer 22for example has a thickness comprised between 20 and 500 nm, preferablybetween 30 and 300 nm, more preferably between 50 and 250 nm, such asfor example from 100 to 150 nm. The second active III-N layer 23 forexample has a thickness comprised between 10 to 100 nm, preferablybetween 20 to 50 nm. Such a combination of thicknesses provides goodcharacteristics for the active layer, for example in terms of the 2DEG21 obtained. The first active III-N layer 22 comprises nitride and oneor more of B, Al, Ga, In and Tl. The first active III-N layer 22 forexample comprises GaN. The second active III-N layer 23 comprisesnitride and one or more of B, Al, Ga, In, and Tl. The second activeIII-N layer 23 for example comprises AlGaN. The term AlGaN relates to acomposition comprising Al, Ga and N in any stochiometric ratio(Al_(x)Ga_(y)N) wherein x is comprised between 0 and 1 and y iscomprised between 0 and 1. Alternatively, the second active III-N layer23 for example comprises AlN. Alternatively, the second active III-Nlayer 23 comprises InAlGaN. A composition such as InAlGaN comprises Inin any suitable amount. Alternatively, both first active III-N layer 22and second active III-N layer 23 comprise InAlGaN, and the second activeIII-N layer 23 comprises a bandgap larger than a bandgap of the firstactive III-N layer 22 and wherein the second active III-N layer 23comprises a polarization larger than the polarization of the firstactive III-N layer 22. Alternatively, both first active III-N layer 22and second active III-N layer 23 comprise BInAlGaN, and the secondactive III-N layer 23 comprises a bandgap larger than a bandgap of thefirst active III-N layer 22 and wherein the second active III-N layer 23comprises a polarization larger than the polarization of the firstactive III-N layer 22. Compositions of the active layer may be chosen inview of characteristics to be obtained, and compositions may varyaccordingly. For example, good results were obtained with a first activeIII-N layer 22 comprising GaN of about 150 nm thickness and a secondactive III-N layer 23 comprising AlGaN of about 20 nm thickness. Apassivation stack 40 is formed on top of the epitaxial III-Nsemiconductor layer stack 20, and more particularly on top of the secondactive III-N layer 23. The passivation stack 40 for example comprisesSiN. The passivation stack 40 comprises SiN with high density, depositedin-situ in an MOCVD reactor. The SiN may be stochiometric ornon-stochiometric. The in-situ SiN may be thickened externally by PECVDor LPCVD SiN or SiO_(x), for example for thicknesses beyond 500 nm,before any other processing takes place. According to an alternativeembodiment, the passivation stack 40 comprises SiO₂. According to afurther alternative embodiment, the passivation stack 40 of FIG. 4Acomprises AlSiN. The Al-doping allows increasing the bandgap of thedielectric material. Alternatively, the passivation stack of FIG. 4Acomprise one or more of Si, Al, O and N. The passivation stack 40 isthen etched away in a gate region 31 and the second active III-N layer23 is partially etched in a gate region 31 on FIG. 4B using thepassivation stack 40 as a mask. In other words, a recess 24 is formed inthe second active III-N layer 23 in the gate region 31. According to analternative embodiment depicted in FIG. 4C, the passivation stack 40 isthen etched away in a gate region 31 and the second active III-N layer23 is fully etched away in a gate region 31 on FIG. 4C using thepassivation stack 40 as a mask. In other words, a recess 24 is formed inthe second active III-N layer 23 in the gate region 31 and completelyextends through the second active III-N layer 23 in the gate region 31,thereby exposing the first active III-N layer 22. This can be achievedby etching in a plasma etching tool such as Reactive Ion Etching or RIEor preferably in an Inductively Coupled Plasma or ICP tool. The reagentgases can be Cl₂ or BCl₃. Alternatively, a digital etching process canbe used, whereas consecutively and iteratively, first the top surface ofthe second active III-N layer is oxidized for example in O₂, O₃ or N₂Oplasma, after which the formed oxide is etched away e.g. in SF₆ or CF₄plasma.

According to an embodiment shown in FIGS. 5A to 5C, a high electronmobility transistor according to the present invention is manufacturedas schematically illustrated by the different steps of FIGS. 5A to 5C.Components having identical reference numbers to components in FIG. 2Ato 2C and FIGS. 3A and 3B and FIGS. 4A to 4C perform the same function.As visible in FIG. 5A, a semiconductor structure 1 according to thepresent invention comprises a substrate 10 and an epitaxial III-Nsemiconductor layer stack 20. The epitaxial III-N semiconductor layerstack 20 comprises a first active III-N layer 22 and a second activeIII-N layer 23, with a two dimensional Electron Gas 21 between the firstactive III-N layer 22 and the second active III-N layer 23. The firstactive III-N layer 22 for example has a thickness comprised between 20and 500 nm, preferably between 30 and 300 nm, more preferably between 50and 250 nm, such as for example from 100 to 150 nm. The second activeIII-N layer 23 for example has a thickness comprised between 10 to 100nm, preferably between 20 to 50 nm. Such a combination of thicknessesprovides good characteristics for the active layer, for example in termsof the 2DEG 21 obtained. The first active III-N layer 22 comprisesnitride and one or more of B, Al, Ga, In and Tl. The first active III-Nlayer 22 for example comprises GaN. The second active III-N layer 23comprises nitride and one or more of B, Al, Ga, In, and Tl. The secondactive III-N layer 23 for example comprises AlGaN. The term AlGaNrelates to a composition comprising Al, Ga and N in any stochiometricratio (Al_(x)Ga_(y)N) wherein x is comprised between 0 and 1 and y iscomprised between 0 and 1. Alternatively, the second active III-N layer23 for example comprises AlN. Alternatively, the second active III-Nlayer 23 comprises InAlGaN. A composition such as InAlGaN comprises Inin any suitable amount. Alternatively, both first active III-N layer 22and second active III-N layer 23 comprise InAlGaN, and the second activeIII-N layer 23 comprises a bandgap larger than a bandgap of the firstactive III-N layer 22 and wherein the second active III-N layer 23comprises a polarization larger than the polarization of the firstactive III-N layer 22. Alternatively, both first active III-N layer 22and second active III-N layer 23 comprise BInAlGaN, and the secondactive III-N layer 23 comprises a bandgap larger than a bandgap of thefirst active III-N layer 22 and wherein the second active III-N layer 23comprises a polarization larger than the polarization of the firstactive III-N layer 22. Compositions of the active layer may be chosen inview of characteristics to be obtained, and compositions may varyaccordingly. For example, good results were obtained with a first activeIII-N layer 22 comprising GaN of about 150 nm thickness and a secondactive III-N layer 23 comprising AlGaN of about 20 nm thickness. Apassivation stack is formed on top of the epitaxial III-N semiconductorlayer stack 20, and more particularly on top of the second active III-Nlayer 23. The passivation stack for example comprises SiN. Thepassivation stack comprises SiN with high density, deposited in-situ inan MOCVD reactor. The SiN may be stochiometric or non-stochiometric. Thein-situ SiN may be thickened externally by PECVD or LPCVD SiN orSiO_(x), for example for thicknesses beyond 500 nm, before any otherprocessing takes place. According to an alternative embodiment, thepassivation stack comprises SiO₂. According to a further alternativeembodiment, the passivation stack comprises AlSiN. The Al-doping allowsincreasing the bandgap of the dielectric material. Alternatively, thepassivation stack comprises one or more of Si, Al, O and N. Thepassivation stack is then etched away in a gate region 31 and the secondactive III-N layer 23 is partially etched in a gate region 31 on FIG. 5Ausing the passivation stack as a mask, thereby forming a recess 24 inthe second active III-N layer 23. In other words, a partial recess 24 isformed in the second active III-N layer 23 in the gate region 31. Thiscan be achieved by etching in a plasma etching tool such as Reactive IonEtching or RIE or preferably in an Inductively Coupled Plasma or ICPtool. The reagent gases can be Cl₂ or BCl₃. Alternatively, a digitaletching process can be used, whereas consecutively and iteratively,first the top surface of the second active III-N layer is oxidized forexample in O₂, O₃ or N₂O plasma, after which the formed oxide is etchedaway e.g. in SF₆ or CF₄ plasma. An electron accepting dielectric layer41 is then formed on top of the second active III-N layer 23 of FIG. 5A,thereby being formed in the recess 24 of the second active III-N layer23. The electron accepting dielectric layer 41 comprises a passivationsurface 410 in contact with the second active III-N layer 23 of theepitaxial III-N semiconductor layer stack 20. The electron acceptingdielectric layer 41 further comprises a dielectric surface 411 oppositeto the passivation surface 410. The second active III-N layer 23comprises a second passivation surface 230 in contact with thepassivation surface 410 of the electron accepting dielectric layer 41,thereby defining a passivation contact interface 231 between the secondactive III-N layer 23 and the electron accepting dielectric layer 41.The passivation contact interface 231 extends such that the passivationsurface 410 is in direct contact with 10 to 30% of the secondpassivation surface in a gate region 31. In other words, the electronaccepting dielectric layer 41 is etched away except in a gate region 31.According to an alternative embodiment, a passivation stack is depositedon top of the second active III-N layer 23 similarly to FIG. 2A to 2C,the passivation stack is then etched away in the gate region 31 and theelectron accepting dielectric layer 41 is then deposited in the gateregion 31 in the partial recess 24, thereby forming the high electronmobility transistor of FIG. 5C. The electron accepting dielectric layer41 comprises for example Mg_(x)Si_(1-x)N, wherein x is comprised between0.05 and 0.95. According to an alternative embodiment, the electronaccepting dielectric layer 41 comprises Mg_(y)Al_(1-y)N, wherein y iscomprised between 0.05 and 0.95. According to a further alternativeembodiment, the electron accepting dielectric layer comprisesMg_(a)Si_(z)Al_(1-a-z)N, wherein a is comprised between 0.05 and 0.95and wherein z is comprised between 0.05 and 0.95 and the a+z iscomprised between 0.1 and 1. The MgSiN or the MgAlN, or the MgSiAlN areepitaxially grown on top of the epitaxial III-N semiconductor layerstack 20, preferably on top of the second active III-N layer 23. Asshown in FIG. 5C, the passivation stack 40 further comprises an oxidelayer 42. The oxide layer 42 also partially extends in the recess 24formed in the second active III-N layer 23. The passivation stack 40 andmore particularly the electron accepting dielectric layer 41 and theoxide layer 42 are for example grown by MOCVD. According to analternative embodiment, the passivation stack 40 is grown by MBE. Theoxide layer 42 for example comprises MgO. According to an alternativeembodiment, the oxide layer 42 comprises AlO_(x) or SiO_(x), or alloysthereof. According to a further alternative embodiment, the oxide layer42 comprises a gate dielectric such as for example HfO_(x), ZrO_(x),etc. The oxide layer 42 comprises an oxide surface 420 in contact withthe dielectric surface 411 and a passivation insulating surface 421opposite to the oxide surface 420. The dielectric surface 411 and theoxide surface 420 extend such that the oxide surface 420 is in directcontact with the dielectric surface 411 along the full surface of thedielectric surface 411. On FIG. 5C, a gate 30 is formed on top of thepassivation stack 40 in the gate region 31. The gate 30 comprises abiasing surface 300 via which a voltage bias is applied to the gate 30and a gate insulating surface 301 opposite to the biasing surface 300.More particularly, a gate is formed in the gate region 31 on top of theoxide layer 42, thereby defining an insulating contact interface 423between the passivation insulating surface 421 and the gate insulatingsurface 301. The insulating contact interface 423 extends such that thegate insulating surface 301 is in direct contact with 100% of thepassivation insulating surface 421. As shown on FIG. 5C, the passivationstack 40 further comprises two electron donating dielectric layers 43;44 formed on top of the second active III-N layer 23 and on both sidesof the electron accepting dielectric layer 41, i.e. on both sides of thegate region 31, such that each of the two electron donating dielectriclayers 43; 44 comprises a III-N contact surface 430; 440 in directcontact with the second active III-N layer 23. The electron donatingdielectric layers 43; 44 comprise SiN with high density, depositedin-situ in an MOCVD reactor. The SiN may be stochiometric ornon-stochiometric. It has been shown experimentally by inventors thatfor example a HEMT structure that is capped with in-situ SiN is notaffected by processing steps, even those that have a high temperaturebudget. According to an alternative embodiment, the electron donatingdielectric layers 43; 44 comprise AlSiN. The Al-doping allows increasingthe bandgap of the dielectric material. According to a furtheralternative embodiment, the electron donating dielectric layers 43; 44comprise one or more of Si, Al, O and N. The electron donatingdielectric layers 43; 44 have a thickness of 1 to 500 nm, preferably 30to 400 nm, more preferably 50 to 300 nm, such as 100 to 200 nm. Thein-situ SiN may be thickened externally by PECVD or LPCVD SiN orSiO_(x), for example for thicknesses beyond 500 nm, before any otherprocessing takes place. On FIG. 5C, the two electron donating dielectriclayers 43; 44 are as thick as the stack of the electron acceptingdielectric layer 41 and of the oxide layer 42. According to analternative embodiment, the two electron donating dielectric layers 43;44 encapsulate the high electron mobility transistor and the twoelectric donating dielectric layer are etched away in a gate region, andare etched away in a gate access region and a drain access regionwherein a source and a drain are then formed. According to analternative embodiment, the two electron donating dielectric layers 43;44 are thicker than the stack of the electron accepting dielectric layer41 and the oxide layer 42. Finally, on FIG. 5C, the passivation stack 40is etched away in a source access region and is etched away in a drainaccess region. More particularly, the two electron donating dielectriclayers 43; 44 of the passivation stack 40 are etched away respectivelyin a source region 51 and in a drain region 52. An ohmic contact is thenformed in the source region 51 and an ohmic contact is then formed inthe drain region 52. Forming an ohmic contact in the source region 51and forming an ohmic contact in the drain region 52 comprise pluralityof process steps. For example, this is done by starting with depositingphotoresist and defining the respective areas of the respective ohmiccontacts with a lithography step. The electron donating dielectriclayers 43; 44 are then partially or fully removed respectively in asource region 51 and in a drain region 52. For example, the electrondonating dielectric layers 43; 44 can be removed by wet etching in HF orbuffered HF or by dry etching in a RIE or ICP plasma tool in a fluorinechemistry. Both dry and wet etches of the electron donating dielectriclayers 43; 44 in a fluorine chemistry will stop on the second activeIII-N layer 23 which acts as an etch-stop with very high selectivity.For example, the etch of the electron donating dielectric layers 43; 44is done in a dry etching system based on fluorine chemistry such as forexample in an inductively coupled plasma system using SF₆ or CF₄ asetching gas and RF, or “platen”, and ICP, or “coil” etching powers of 10W to 150 W respectively. This allows for thorough removal of theremaining electron donating dielectric layer 43; 44 without removing thesecond active III-N layer 23 or any of the layers below. According to analternative embodiment, the second active III-N layer 23 is partiallyetched in a wet etch, for example in an alkaline solution or in resistdeveloper, thereby allowing to form respective ohmic contacts in asource region 51 and in a drain region 52 partly in the second activeIII-N layer 23. Once the areas of the ohmic contacts are defined, i.e.when the source region 51 and the drain region 52 have been defined, ametal layer or a stack of metal layers can be deposited, for example bythermal evaporation, or by sputtering, or by e-beam evaporation. Metalpatterns are consecutively defined by performing lift-off of the metal,on top of the photoresist and not in contact with the second activeIII-N layer 23. Alternatively, the photoresist is first removed and themetal stack comprising for example Ti and Al is deposited and then asecond photoresist deposition and photolithography steps are performedto allow dry etching of the metal stack in areas where it is unwantedand removing the photoresist. The defined ohmic contacts may then besubjected to one or more alloying steps, for example a rapid thermalannealing step for a duration of one minute in a reduced or inertatmosphere such as for example hydrogen or forming gas or nitrogen gasat a temperature for example between 800° C. and 900° C. A high electronmobility transistor 1 according to the present invention is obtained.

According to an embodiment shown in FIGS. 6A to 6C, a high electronmobility transistor according to the present invention is manufacturedas schematically illustrated by the different steps of FIGS. 2A to 2C.Components having identical reference numbers to components in FIG. 2Ato 2C and FIGS. 3A and 3B and FIGS. 4A to 4C and FIG. 5A to 5C performthe same function. FIGS. 6A to 6C illustrate an alternativemanufacturing method of a semiconductor structure 1 according to thepresent invention. On FIG. 6A, a semiconductor structure according tothe present invention comprises a substrate 10 and an epitaxial III-Nsemiconductor layer stack 20. The epitaxial III-N semiconductor layerstack 20 comprises a first active III-N layer 22 and a second activeIII-N layer 23, with a two dimensional Electron Gas 21 between the firstactive III-N layer 22 and the second active III-N layer 23. The firstactive III-N layer 22 for example has a thickness comprised between 20and 500 nm, preferably between 30 and 300 nm, more preferably between 50and 250 nm, such as for example from 100 to 150 nm. The second activeIII-N layer 23 for example has a thickness comprised between 10 to 100nm, preferably between 20 to 50 nm. Such a combination of thicknessesprovides good characteristics for the active layer, for example in termsof the 2DEG 21 obtained. The first active III-N layer 22 comprisesnitride and one or more of B, Al, Ga, In and Tl. The first active III-Nlayer 22 for example comprises GaN. The second active III-N layer 23comprises nitride and one or more of B, Al, Ga, In, and Tl. The secondactive III-N layer 23 for example comprises AlGaN. The term AlGaNrelates to a composition comprising Al, Ga and N in any stochiometricratio (Al_(x)Ga_(y)N) wherein x is comprised between 0 and 1 and y iscomprised between 0 and 1. Alternatively, the second active III-N layer23 for example comprises AlN. Alternatively, the second active III-Nlayer 23 comprises InAlGaN. A composition such as InAlGaN comprises Inin any suitable amount. Alternatively, both first active III-N layer 22and second active III-N layer 23 comprise InAlGaN, and the second activeIII-N layer 23 comprises a bandgap larger than a bandgap of the firstactive III-N layer 22 and wherein the second active III-N layer 23comprises a polarization larger than the polarization of the firstactive III-N layer 22. Alternatively, both first active III-N layer 22and second active III-N layer 23 comprise BInAlGaN, and the secondactive III-N layer 23 comprises a bandgap larger than a bandgap of thefirst active III-N layer 22 and wherein the second active III-N layer 23comprises a polarization larger than the polarization of the firstactive III-N layer 22. Compositions of the active layer may be chosen inview of characteristics to be obtained, and compositions may varyaccordingly. For example, good results were obtained with a first activeIII-N layer 22 comprising GaN of about 150 nm thickness and a secondactive III-N layer 23 comprising AlGaN of about 20 nm thickness. Asvisible on FIG. 6B, a passivation stack 40 is formed on top of theepitaxial semiconductor layer stack 20. The passivation stack 40comprises an electron donating dielectric layer. The passivation stack40 for example comprises SiN. The passivation stack 40 comprises SiNwith high density, deposited in-situ in an MOCVD reactor. The SiN may bestochiometric or non-stochiometric. The in-situ SiN may be thickenedexternally by PECVD or LPCVD SiN or SiO_(x), for example for thicknessesbeyond 500 nm, before any other processing takes place. According to analternative embodiment, the passivation stack 40 comprises SiO₂.According to a further alternative embodiment, the passivation stack 40of FIG. 6B comprises AlSiN. The Al-doping allows increasing the bandgapof the dielectric material. Alternatively, the passivation stack of FIG.6B comprise one or more of Si, Al, O and N. The passivation stack 40 isthen etched away in a gate region 31, thereby exposing the secondpassivation surface 230 of the second active III-N layer 23, and therebyforming two electron donating dielectric layers 43; 44 on top of thesecond active III-N layer 23 and on both sides of the gate region 31,such that each of the two electron donating dielectric layers 43; 44comprises a III-N contact surface 430; 440 in direct contact with thesecond active III-N layer 23. The second active III-N layer 23 ispartially etched in a gate region 31 on FIG. 6B using the passivationstack as a mask, thereby forming a recess 24 in the second active III-Nlayer 23. In other words, a partial recess 24 is formed in the secondactive III-N layer 23 in the gate region 31. This can be achieved byetching in a plasma etching tool such as Reactive Ion Etching or RIE orpreferably in an Inductively Coupled Plasma or ICP tool. The reagentgases can be Cl₂ or BCl₃. Alternatively, a digital etching process canbe used, whereas consecutively and iteratively, first the top surface ofthe second active III-N layer is oxidized for example in O₂, O₃ or N₂Oplasma, after which the formed oxide is etched away e.g. in SF₆ or CF₄plasma. As visible in FIG. 6B, an electron accepting dielectric layer 41is then formed in the gate region 31. According to an alternativeembodiment, the electron accepting dielectric layer 41 is deposited overthe two electron donating dielectric layers 43; 44 and over the recess24 in the gate region and the electron accepting dielectric layer 41 isthen etched away except in the gate region 31, thereby remaining in therecess 24 as depicted in FIG. 6C. The electron accepting dielectriclayer 41 comprises a passivation surface 410 in contact with the secondactive III-N layer 23 of the epitaxial semiconductor layer stack 20. Theelectron accepting dielectric layer 41 further comprises a dielectricsurface 411 opposite to the passivation surface 410. The second activeIII-N layer 23 comprises a second passivation surface 230 in contactwith the passivation surface 410 of the electron accepting dielectriclayer 41, thereby defining a passivation contact interface 231 betweenthe second active III-N layer 23 and the electron accepting dielectriclayer 41. The passivation contact interface 231 extends such that thepassivation surface 410 is in direct contact with 10 to 30% of thesecond passivation surface in a gate region 31. The thickness of theelectron accepting dielectric layer is lower than the depth of therecess 24 formed in the second active III-N layer 23. The electronaccepting dielectric layer 41 comprises for example Mg_(x)Si_(1-x)N,wherein x is comprised between 0.05 and 0.95. According to analternative embodiment, the electron accepting dielectric layer 41comprises Mg_(y)Al_(1-y)N, wherein y is comprised between 0.05 and 0.95.According to a further alternative embodiment, the electron acceptingdielectric layer comprises Mg_(a)Si_(z)Al_(1-a-z)N, wherein a iscomprised between 0.05 and 0.95 and wherein z is comprised between 0.05and 0.95 and the a+z is comprised between 0.1 and 1. The MgSiN or theMgAlN, or the MgSiAlN are epitaxially grown on top of the epitaxialIII-N semiconductor layer stack 20, preferably on top of the secondactive III-N layer 23. As shown in FIG. 6C, the passivation stack 40further comprises an oxide layer 42. The oxide layer 42 also partiallyextends in the recess 24 formed in the second active III-N layer 23. Thepassivation stack 40 and more particularly the electron acceptingdielectric layer 41 and the oxide layer 42 are for example grown byMOCVD. According to an alternative embodiment, the passivation stack 40is grown by MBE. The oxide layer 42 for example comprises MgO. Accordingto an alternative embodiment, the oxide layer 42 comprises AlO_(x) orSiO_(x), or alloys thereof. According to a further alternativeembodiment, the oxide layer 42 comprises a gate dielectric such as forexample HfO_(x), ZrO_(x), etc. The oxide layer 42 comprises an oxidesurface 420 in contact with the dielectric surface 411 and a passivationinsulating surface 421 opposite to the oxide surface 420. The dielectricsurface 411 and the oxide surface 420 extend such that the oxide surface420 is in direct contact with the dielectric surface 411 along the fullsurface of the dielectric surface 411. On FIG. 6C, a gate 30 is formedon top of the passivation stack 40 in the gate region 31. The gate 30comprises a biasing surface 300 via which a voltage bias is applied tothe gate 30 and a gate insulating surface 301 opposite to the biasingsurface 300. More particularly, a gate is formed in the gate region 31on top of the oxide layer 42, thereby defining an insulating contactinterface 423 between the passivation insulating surface 421 and thegate insulating surface 301. The insulating contact interface 423extends such that the gate insulating surface 301 is in direct contactwith 100% of the passivation insulating surface 421. As shown on FIG.6C, the passivation stack 40 further comprises two electron donatingdielectric layers 43; 44 formed on top of the second active III-N layer23 and on both sides of the electron accepting dielectric layer 41, i.e.on both sides of the gate region 31, such that each of the two electrondonating dielectric layers 43; 44 comprises a III-N contact surface 430;440 in direct contact with the second active III-N layer 23. Theelectron donating dielectric layers 43; 44 comprise SiN with highdensity, deposited in-situ in an MOCVD reactor. The SiN may bestochiometric or non-stochiometric. It has been shown experimentally byinventors that for example a HEMT structure that is capped with in-situSiN is not affected by processing steps, even those that have a hightemperature budget. According to an alternative embodiment, the electrondonating dielectric layers 43; 44 comprise AlSiN. The Al-doping allowsincreasing the bandgap of the dielectric material. According to afurther alternative embodiment, the electron donating dielectric layers43; 44 comprise one or more of Si, Al, O and N. The electron donatingdielectric layers 43; 44 have a thickness of 1 to 500 nm, preferably 30to 400 nm, more preferably 50 to 300 nm, such as 100 to 200 nm. Thein-situ SiN may be thickened externally by PECVD or LPCVD SiN orSiO_(x), for example for thicknesses beyond 500 nm, before any otherprocessing takes place. On FIG. 6C, the two electron donating dielectriclayers 43; 44 are as thick as the stack of the electron acceptingdielectric layer 41 and of the oxide layer 42. According to analternative embodiment, the two electron donating dielectric layers 43;44 encapsulate the high electron mobility transistor and the twoelectric donating dielectric layer are etched away in a gate region, andare etched away in a gate access region and a drain access regionwherein a source and a drain are then formed. According to analternative embodiment, the two electron donating dielectric layers 43;44 are thicker than the stack of the electron accepting dielectric layer41 and the oxide layer 42. Finally, on FIG. 6C, the passivation stack 40is etched away in a source access region and is etched away in a drainaccess region. More particularly, the two electron donating dielectriclayers 43; 44 of the passivation stack 40 are etched away respectivelyin a source region 51 and in a drain region 52. An ohmic contact is thenformed in the source region 51 and an ohmic contact is then formed inthe drain region 52. Forming an ohmic contact in the source region 51and forming an ohmic contact in the drain region 52 comprise pluralityof process steps. For example, this is done by starting with depositingphotoresist and defining the respective areas of the respective ohmiccontacts with a lithography step. The electron donating dielectriclayers 43; 44 are then partially or fully removed respectively in asource region 51 and in a drain region 52. For example, the electrondonating dielectric layers 43; 44 can be removed by wet etching in HF orbuffered HF or by dry etching in a RIE or ICP plasma tool in a fluorinechemistry. Both dry and wet etches of the electron donating dielectriclayers 43; 44 in a fluorine chemistry will stop on the second activeIII-N layer 23 which acts as an etch-stop with very high selectivity.For example, the etch of the electron donating dielectric layers 43; 44is done in a dry etching system based on fluorine chemistry such as forexample in an inductively coupled plasma system using SF₆ or CF₄ asetching gas and RF, or “platen”, and ICP, or “coil” etching powers of 10W to 150 W respectively. This allows for thorough removal of theremaining electron donating dielectric layer 43; 44 without removing thesecond active III-N layer 23 or any of the layers below. According to analternative embodiment, the second active III-N layer 23 is partiallyetched in a wet etch, for example in an alkaline solution or in resistdeveloper, thereby allowing to form respective ohmic contacts in asource region 51 and in a drain region 52 partly in the second activeIII-N layer 23. Once the areas of the ohmic contacts are defined, i.e.when the source region 51 and the drain region 52 have been defined, ametal layer or a stack of metal layers can be deposited, for example bythermal evaporation, or by sputtering, or by e-beam evaporation. Metalpatterns are consecutively defined by performing lift-off of the metal,on top of the photoresist and not in contact with the second activeIII-N layer 23. Alternatively, the photoresist is first removed and themetal stack comprising for example Ti and Al is deposited and then asecond photoresist deposition and photolithography steps are performedto allow dry etching of the metal stack in areas where it is unwantedand removing the photoresist. The defined ohmic contacts may then besubjected to one or more alloying steps, for example a rapid thermalannealing step for a duration of one minute in a reduced or inertatmosphere such as for example hydrogen or forming gas or nitrogen gasat a temperature for example between 800° C. and 900° C. A high electronmobility transistor 1 according to the present invention is obtained.

According to an embodiment shown in FIGS. 7A to 7C, a high electronmobility transistor according to the present invention is manufacturedas schematically illustrated by the different steps of FIGS. 7A to 7C.Components having identical reference numbers to components in FIG. 2Ato 2C and FIGS. 3A and 3B and FIGS. 4A to 4C and FIGS. 5A to 5C andFIGS. 6A to 6C perform the same function. As visible in FIG. 7A, asemiconductor structure 1 according to the present invention comprises asubstrate 10 and an epitaxial III-N semiconductor layer stack 20. Theepitaxial III-N semiconductor layer stack 20 comprises a first activeIII-N layer 22 and a second active III-N layer 23, with a twodimensional Electron Gas 21 between the first active III-N layer 22 andthe second active III-N layer 23. The first active III-N layer 22 forexample has a thickness comprised between 20 and 500 nm, preferablybetween 30 and 300 nm, more preferably between 50 and 250 nm, such asfor example from 100 to 150 nm. The second active III-N layer 23 forexample has a thickness comprised between 10 to 100 nm, preferablybetween 20 to 50 nm. Such a combination of thicknesses provides goodcharacteristics for the active layer, for example in terms of the 2DEG21 obtained. The first active III-N layer 22 comprises nitride and oneor more of B, Al, Ga, In and Tl. The first active III-N layer 22 forexample comprises GaN. The second active III-N layer 23 comprisesnitride and one or more of B, Al, Ga, In, and Tl. The second activeIII-N layer 23 for example comprises AlGaN. The term AlGaN relates to acomposition comprising Al, Ga and N in any stochiometric ratio(Al_(x)Ga_(y)N) wherein x is comprised between 0 and 1 and y iscomprised between 0 and 1. Alternatively, the second active III-N layer23 for example comprises AlN. Alternatively, the second active III-Nlayer 23 comprises InAlGaN. A composition such as InAlGaN comprises Inin any suitable amount. Alternatively, both first active III-N layer 22and second active III-N layer 23 comprise InAlGaN, and the second activeIII-N layer 23 comprises a bandgap larger than a bandgap of the firstactive III-N layer 22 and wherein the second active III-N layer 23comprises a polarization larger than the polarization of the firstactive III-N layer 22. Alternatively, both first active III-N layer 22and second active III-N layer 23 comprise BInAlGaN, and the secondactive III-N layer 23 comprises a bandgap larger than a bandgap of thefirst active III-N layer 22 and wherein the second active III-N layer 23comprises a polarization larger than the polarization of the firstactive III-N layer 22. Compositions of the active layer may be chosen inview of characteristics to be obtained, and compositions may varyaccordingly. For example, good results were obtained with a first activeIII-N layer 22 comprising GaN of about 150 nm thickness and a secondactive III-N layer 23 comprising AlGaN of about 20 nm thickness. Apartial recess 24 is formed in the second active III-N layer 23 in agate region 31. This can be achieved by etching in a plasma etching toolsuch as Reactive Ion Etching or RIE or preferably in an InductivelyCoupled Plasma or ICP tool. The reagent gases can be Cl₂ or BCl₃.Alternatively, a digital etching process can be used, whereasconsecutively and iteratively, first the top surface of the secondactive III-N layer is oxidized for example in O₂, O₃ or N₂O plasma,after which the formed oxide is etched away e.g. in SF₆ or CF₄ plasma.An electron accepting dielectric layer 41 is formed on top of theepitaxial III-N semiconductor layer stack 20, and more particularly ontop of the second active III-N layer 23, thereby being formed in therecess 24 of the second active III-N layer 23. According to analternative embodiment, a mask is deposited in the second active III-Nlayer 23 and the mask is etched away in the gate region 31. The electronaccepting dielectric layer 41 is then formed in the gate region 31, asdepicted in FIG. 7B. The electron accepting dielectric layer 41comprises a passivation surface 410 in contact with the second activeIII-N layer 23 of the epitaxial III-N semiconductor layer stack 20. Theelectron accepting dielectric layer 41 further comprises a dielectricsurface 411 opposite to the passivation surface 410. The second activeIII-N layer 23 comprises a second passivation surface 230 in contactwith the passivation surface 410 of the electron accepting dielectriclayer 41, thereby defining a passivation contact interface 231 betweenthe second active III-N layer 23 and the electron accepting dielectriclayer 41. The passivation contact interface 231 extends such that thepassivation surface 410 is in direct contact with 10 to 30% of thesecond passivation surface in a gate region 31. In other words, theelectron accepting dielectric layer 41 is etched away except in a gateregion 31. The thickness of the electron accepting dielectric layer islarger than the depth of the recess 24 formed in the second active III-Nlayer 23. The electron accepting dielectric layer 41 comprises forexample Mg_(x)Si_(1-x)N, wherein x is comprised between 0.05 and 0.95.According to an alternative embodiment, the electron acceptingdielectric layer 41 comprises Mg_(y)Al_(1-y)N, wherein y is comprisedbetween 0.05 and 0.95. According to a further alternative embodiment,the electron accepting dielectric layer comprisesMg_(a)Si_(z)Al_(1-a-z)N, wherein a is comprised between 0.05 and 0.95and wherein z is comprised between 0.05 and 0.95 and the a+z iscomprised between 0.1 and 1. The MgSiN or the MgAlN, or the MgSiAlN areepitaxially grown on top of the epitaxial III-N semiconductor layerstack 20, preferably on top of the second active III-N layer 23. Asshown in FIG. 7B, the passivation stack 40 further comprises an oxidelayer 42. The passivation stack 40 and more particularly the electronaccepting dielectric layer 41 and the oxide layer 42 are for examplegrown by MOCVD. According to an alternative embodiment, the passivationstack 40 is grown by MBE. The oxide layer 42 for example comprises MgO.According to an alternative embodiment, the oxide layer 42 comprisesAlO_(x) or SiO_(x), or alloys thereof. According to a furtheralternative embodiment, the oxide layer 42 comprises a gate dielectricsuch as for example HfO_(x), ZrO_(x), etc. The oxide layer 42 comprisesan oxide surface 420 in contact with the dielectric surface 411 and apassivation insulating surface 421 opposite to the oxide surface 420.The dielectric surface 411 and the oxide surface 420 extend such thatthe oxide surface 420 is in direct contact with the dielectric surface411 along the full surface of the dielectric surface 411. On FIG. 7B, agate 30 is formed on top of the passivation stack 40 in the gate region31. The gate 30 comprises a biasing surface 300 via which a voltage biasis applied to the gate 30 and a gate insulating surface 301 opposite tothe biasing surface 300. More particularly, a gate is formed in the gateregion 31 on top of the oxide layer 42, thereby defining an insulatingcontact interface 423 between the passivation insulating surface 421 andthe gate insulating surface 301. The insulating contact interface 423extends such that the gate insulating surface 301 is in direct contactwith 100% of the passivation insulating surface 421. As shown on FIG.7B, the passivation stack 40 further comprises two electron donatingdielectric layers 43; 44 formed on top of the second active III-N layer23 and on both sides of the electron accepting dielectric layer 41, i.e.on both sides of the gate region 31, such that each of the two electrondonating dielectric layers 43; 44 comprises a III-N contact surface 430;440 in direct contact with the second active III-N layer 23. Theelectron donating dielectric layers 43; 44 comprise SiN with highdensity, deposited in-situ in an MOCVD reactor. The SiN may bestochiometric or non-stochiometric. It has been shown experimentally byinventors that for example a HEMT structure that is capped with in-situSiN is not affected by processing steps, even those that have a hightemperature budget. According to an alternative embodiment, the electrondonating dielectric layers 43; 44 comprise AlSiN. The Al-doping allowsincreasing the bandgap of the dielectric material. According to afurther alternative embodiment, the electron donating dielectric layers43; 44 comprise one or more of Si, Al, O and N. The electron donatingdielectric layers 43; 44 have a thickness of 1 to 500 nm, preferably 30to 400 nm, more preferably 50 to 300 nm, such as 100 to 200 nm. Thein-situ SiN may be thickened externally by PECVD or LPCVD SiN orSiO_(x), for example for thicknesses beyond 500 nm, before any otherprocessing takes place. On FIG. 7C, the two electron donating dielectriclayers 43; 44 are as thick as the stack of the electron acceptingdielectric layer 41 and of the oxide layer 42. According to analternative embodiment, the two electron donating dielectric layers 43;44 encapsulate the high electron mobility transistor and the twoelectric donating dielectric layer are etched away in a gate region, andare etched away in a gate access region and a drain access regionwherein a source and a drain are then formed. According to analternative embodiment, the two electron donating dielectric layers 43;44 are thicker than the stack of the electron accepting dielectric layer41 and the oxide layer 42. Finally, on FIG. 7B, the passivation stack 40is etched away in a source access region and is etched away in a drainaccess region. More particularly, the two electron donating dielectriclayers 43; 44 of the passivation stack 40 are etched away respectivelyin a source region 51 and in a drain region 52. An ohmic contact is thenformed in the source region 51 and an ohmic contact is then formed inthe drain region 52. Forming an ohmic contact in the source region 51and forming an ohmic contact in the drain region 52 comprise pluralityof process steps. For example, this is done by starting with depositingphotoresist and defining the respective areas of the respective ohmiccontacts with a lithography step. The electron donating dielectriclayers 43; 44 are then partially or fully removed respectively in asource region 51 and in a drain region 52. For example, the electrondonating dielectric layers 43; 44 can be removed by wet etching in HF orbuffered HF or by dry etching in a RIE or ICP plasma tool in a fluorinechemistry. Both dry and wet etches of the electron donating dielectriclayers 43; 44 in a fluorine chemistry will stop on the second activeIII-N layer 23 which acts as an etch-stop with very high selectivity.For example, the etch of the electron donating dielectric layers 43; 44is done in a dry etching system based on fluorine chemistry such as forexample in an inductively coupled plasma system using SF₆ or CF₄ asetching gas and RF, or “platen”, and ICP, or “coil” etching powers of 10W to 150 W respectively. This allows for thorough removal of theremaining electron donating dielectric layer 43; 44 without removing thesecond active III-N layer 23 or any of the layers below. According to analternative embodiment, the second active III-N layer 23 is partiallyetched in a wet etch, for example in an alkaline solution or in resistdeveloper, thereby allowing to form respective ohmic contacts in asource region 51 and in a drain region 52 partly in the second activeIII-N layer 23. Once the areas of the ohmic contacts are defined, i.e.when the source region 51 and the drain region 52 have been defined, ametal layer or a stack of metal layers can be deposited, for example bythermal evaporation, or by sputtering, or by e-beam evaporation. Metalpatterns are consecutively defined by performing lift-off of the metal,on top of the photoresist and not in contact with the second activeIII-N layer 23. Alternatively, the photoresist is first removed and themetal stack comprising for example Ti and Al is deposited and then asecond photoresist deposition and photolithography steps are performedto allow dry etching of the metal stack in areas where it is unwantedand removing the photoresist. The defined ohmic contacts may then besubjected to one or more alloying steps, for example a rapid thermalannealing step for a duration of one minute in a reduced or inertatmosphere such as for example hydrogen or forming gas or nitrogen gasat a temperature for example between 800° C. and 900° C. A high electronmobility transistor 1 according to the present invention is obtained.

According to an embodiment shown in FIGS. 8A to 8C, a high electronmobility transistor according to the present invention is manufacturedas schematically illustrated by the different steps of FIGS. 8A to 8C.Components having identical reference numbers to components in FIG. 2Ato 2C and FIGS. 3A and 3B and FIGS. 4A to 4C and FIGS. 5A to 5C andFIGS. 6A to 6C and FIGS. 7A to 7C perform the same function. As visiblein FIG. 8A, a semiconductor structure according to the present inventioncomprises a substrate 10 and an epitaxial III-N semiconductor layerstack 20. The epitaxial III-N semiconductor layer stack 20 comprises afirst active III-N layer 22 and a second active III-N layer 23, with atwo dimensional Electron Gas 21 between the first active III-N layer 22and the second active III-N layer 23. The first active III-N layer 22for example has a thickness comprised between 20 and 500 nm, preferablybetween 30 and 300 nm, more preferably between 50 and 250 nm, such asfor example from 100 to 150 nm. The second active III-N layer 23 forexample has a thickness comprised between 10 to 100 nm, preferablybetween 20 to 50 nm. Such a combination of thicknesses provides goodcharacteristics for the active layer, for example in terms of the 2DEG21 obtained. The first active III-N layer 22 comprises nitride and oneor more of B, Al, Ga, In and Tl. The first active III-N layer 22 forexample comprises GaN. The second active III-N layer 23 comprisesnitride and one or more of B, Al, Ga, In, and Tl. The second activeIII-N layer 23 for example comprises AlGaN. The term AlGaN relates to acomposition comprising Al, Ga and N in any stochiometric ratio(Al_(x)Ga_(y)N) wherein x is comprised between 0 and 1 and y iscomprised between 0 and 1. Alternatively, the second active III-N layer23 for example comprises AlN. Alternatively, the second active III-Nlayer 23 comprises InAlGaN. A composition such as InAlGaN comprises Inin any suitable amount. Alternatively, both first active III-N layer 22and second active III-N layer 23 comprise InAlGaN, and the second activeIII-N layer 23 comprises a bandgap larger than a bandgap of the firstactive III-N layer 22 and wherein the second active III-N layer 23comprises a polarization larger than the polarization of the firstactive III-N layer 22. Alternatively, both first active III-N layer 22and second active III-N layer 23 comprise BInAlGaN, and the secondactive III-N layer 23 comprises a bandgap larger than a bandgap of thefirst active III-N layer 22 and wherein the second active III-N layer 23comprises a polarization larger than the polarization of the firstactive III-N layer 22. Compositions of the active layer may be chosen inview of characteristics to be obtained, and compositions may varyaccordingly. For example, good results were obtained with a first activeIII-N layer 22 comprising GaN of about 150 nm thickness and a secondactive III-N layer 23 comprising AlGaN of about 20 nm thickness. A fullrecess 24 is formed in the second active III-N layer 23 in a gate region31, thereby exposing the first active III-N layer 22. This can beachieved by etching in a plasma etching tool such as Reactive IonEtching or RIE or preferably in an Inductively Coupled Plasma or ICPtool. The reagent gases can be Cl₂ or BCl₃. Alternatively, a digitaletching process can be used, whereas consecutively and iteratively,first the top surface of the second active III-N layer is oxidized forexample in O₂, O₃ or N₂O plasma, after which the formed oxide is etchedaway e.g. in SF₆ or CF₄ plasma. An electron accepting dielectric layer41 is formed on top of the epitaxial III-N semiconductor layer stack 20,and more particularly on top of the second active III-N layer 23,thereby being formed in the recess 24 of the second active III-N layer23. According to an alternative embodiment, a mask is deposited on topof the second active III-N layer 23 and the mask is etched away in thegate region 31. The electron accepting dielectric layer 41 is thenformed in the gate region 31 as depicted in FIG. 8C. The electronaccepting dielectric layer 41 comprises a passivation surface 410 incontact with the second active III-N layer 23 of the epitaxial III-Nsemiconductor layer stack 20. The electron accepting dielectric layer 41further comprises a dielectric surface 411 opposite to the passivationsurface 410. The second active III-N layer 23 comprises a secondpassivation surface 230 in contact with the passivation surface 410 ofthe electron accepting dielectric layer 41, thereby defining apassivation contact interface 231 between the second active III-N layer23 and the electron accepting dielectric layer 41. The passivationcontact interface 231 extends such that the passivation surface 410 isin direct contact with 10 to 30% of the second passivation surface in agate region 31. In other words, the electron accepting dielectric layer41 is etched away except in a gate region 31. The thickness of theelectron accepting dielectric layer is smaller than the depth of therecess 24 formed in the second active III-N layer 23. The electronaccepting dielectric layer 41 comprises for example Mg_(x)Si_(1-x)N,wherein x is comprised between 0.05 and 0.95. According to analternative embodiment, the electron accepting dielectric layer 41comprises Mg_(y)Al_(1-y)N, wherein y is comprised between 0.05 and 0.95.According to a further alternative embodiment, the electron acceptingdielectric layer comprises Mg_(a)Si_(z)Al_(1-a-z)N, wherein a iscomprised between 0.05 and 0.95 and wherein z is comprised between 0.05and 0.95 and the a+z is comprised between 0.1 and 1. The MgSiN or theMgAlN, or the MgSiAlN are epitaxially grown on top of the epitaxialIII-N semiconductor layer stack 20, preferably on top of the secondactive III-N layer 23. As shown in FIG. 8C, the passivation stack 40further comprises an oxide layer 42. The passivation stack 40 and moreparticularly the electron accepting dielectric layer 41 and the oxidelayer 42 are for example grown by MOCVD. According to an alternativeembodiment, the passivation stack 40 is grown by MBE. The oxide layer 42for example comprises MgO. According to an alternative embodiment, theoxide layer 42 comprises AlO_(x) or SiO_(x), or alloys thereof.According to a further alternative embodiment, the oxide layer 42comprises a gate dielectric such as for example HfO_(x), ZrO_(x), etc.The oxide layer 42 also extends in the recess 24 formed in the secondactive III-N layer 23. The oxide layer 42 comprises an oxide surface 420in contact with the dielectric surface 411 and a passivation insulatingsurface 421 opposite to the oxide surface 420. The dielectric surface411 and the oxide surface 420 extend such that the oxide surface 420 isin direct contact with the dielectric surface 411 along the full surfaceof the dielectric surface 411. On FIG. 8C, a gate 30 is formed on top ofthe passivation stack 40 in the gate region 31. The gate 30 comprises abiasing surface 300 via which a voltage bias is applied to the gate 30and a gate insulating surface 301 opposite to the biasing surface 300.More particularly, a gate is formed in the gate region 31 on top of theoxide layer 42, thereby defining an insulating contact interface 423between the passivation insulating surface 421 and the gate insulatingsurface 301. The insulating contact interface 423 extends such that thegate insulating surface 301 is in direct contact with 100% of thepassivation insulating surface 421. Ohmic contacts could be formed in asource region and a drain region as described in FIG. 8C, therebyforming a metal-oxide-semiconductor field-effect transistor 1.

According to an embodiment shown in FIGS. 9A to 9C, a high electronmobility transistor according to the present invention is manufacturedas schematically illustrated by the different steps of FIGS. 9A to 9C.Components having identical reference numbers to components in FIG. 2Ato 2C and FIGS. 3A and 3B and FIGS. 4A to 4C and FIGS. 5A to 5C andFIGS. 6A to 6C and FIGS. 7A to 7C perform the same function. As visiblein FIG. 9A, a semiconductor structure according to the present inventioncomprises a substrate 10 and an epitaxial III-N semiconductor layerstack 20. The epitaxial III-N semiconductor layer stack 20 comprises afirst active III-N layer 22 and a second active III-N layer 23, with atwo dimensional Electron Gas 21 between the first active III-N layer 22and the second active III-N layer 23. The first active III-N layer 22for example has a thickness comprised between 20 and 500 nm, preferablybetween 30 and 300 nm, more preferably between 50 and 250 nm, such asfor example from 100 to 150 nm. The second active III-N layer 23 forexample has a thickness comprised between 10 to 100 nm, preferablybetween 20 to 50 nm. Such a combination of thicknesses provides goodcharacteristics for the active layer, for example in terms of the 2DEG21 obtained. The first active III-N layer 22 comprises nitride and oneor more of B, Al, Ga, In and Tl. The first active III-N layer 22 forexample comprises GaN. The second active III-N layer 23 comprisesnitride and one or more of B, Al, Ga, In, and Tl. The second activeIII-N layer 23 for example comprises AlGaN. The term AlGaN relates to acomposition comprising Al, Ga and N in any stochiometric ratio(Al_(x)Ga_(y)N) wherein x is comprised between 0 and 1 and y iscomprised between 0 and 1. Alternatively, the second active III-N layer23 for example comprises AlN. Alternatively, the second active III-Nlayer 23 comprises InAlGaN. A composition such as InAlGaN comprises Inin any suitable amount. Alternatively, both first active III-N layer 22and second active III-N layer 23 comprise InAlGaN, and the second activeIII-N layer 23 comprises a bandgap larger than a bandgap of the firstactive III-N layer 22 and wherein the second active III-N layer 23comprises a polarization larger than the polarization of the firstactive III-N layer 22. Alternatively, both first active III-N layer 22and second active III-N layer 23 comprise BInAlGaN, and the secondactive III-N layer 23 comprises a bandgap larger than a bandgap of thefirst active III-N layer 22 and wherein the second active III-N layer 23comprises a polarization larger than the polarization of the firstactive III-N layer 22. Compositions of the active layer may be chosen inview of characteristics to be obtained, and compositions may varyaccordingly. For example, good results were obtained with a first activeIII-N layer 22 comprising GaN of about 150 nm thickness and a secondactive III-N layer 23 comprising AlGaN of about 20 nm thickness. A fullrecess 24 is formed in the second active III-N layer 23 in a gate region31, thereby exposing the first active III-N layer 22. This can beachieved by etching in a plasma etching tool such as Reactive IonEtching or RIE or preferably in an Inductively Coupled Plasma or ICPtool. The reagent gases can be Cl₂ or BCl₃. Alternatively, a digitaletching process can be used, whereas consecutively and iteratively,first the top surface of the second active III-N layer is oxidized forexample in O₂, O₃ or N₂O plasma, after which the formed oxide is etchedaway e.g. in SF₆ or CF₄ plasma. An electron accepting dielectric layer41 is formed on top of the epitaxial III-N semiconductor layer stack 20,and more particularly on top of the second active III-N layer 23,thereby being formed in the recess 24 of the second active III-N layer23. According to an alternative embodiment, a mask is deposited on topof the second active III-N layer 23 and the mask is etched away in thegate region 31. The electron accepting dielectric layer 41 is thenformed in the gate region 31 as depicted in FIG. 9B. The electronaccepting dielectric layer 41 comprises a passivation surface 410 incontact with the second active III-N layer 23 of the epitaxial III-Nsemiconductor layer stack 20. The electron accepting dielectric layer 41further comprises a dielectric surface 411 opposite to the passivationsurface 410. The second active III-N layer 23 comprises a secondpassivation surface 230 in contact with the passivation surface 410 ofthe electron accepting dielectric layer 41, thereby defining apassivation contact interface 231 between the second active III-N layer23 and the electron accepting dielectric layer 41. The passivationcontact interface 231 extends such that the passivation surface 410 isin direct contact with 10 to 30% of the second passivation surface in agate region 31. In other words, the electron accepting dielectric layer41 is etched away except in a gate region 31. The thickness of theelectron accepting dielectric layer is larger than the depth of therecess 24 formed in the second active III-N layer 23. The electronaccepting dielectric layer 41 comprises for example Mg_(x)Si_(1-x)N,wherein x is comprised between 0.05 and 0.95. According to analternative embodiment, the electron accepting dielectric layer 41comprises Mg_(y)Al_(1-y)N, wherein y is comprised between 0.05 and 0.95.According to a further alternative embodiment, the electron acceptingdielectric layer comprises Mg_(a)Si_(z)Al_(1-a-z)N, wherein a iscomprised between 0.05 and 0.95 and wherein z is comprised between 0.05and 0.95 and the a+z is comprised between 0.1 and 1. The MgSiN or theMgAlN, or the MgSiAlN are epitaxially grown on top of the epitaxialIII-N semiconductor layer stack 20, preferably on top of the secondactive III-N layer 23. As shown in FIG. 9C, the passivation stack 40further comprises an oxide layer 42. The passivation stack 40 and moreparticularly the electron accepting dielectric layer 41 and the oxidelayer 42 are for example grown by MOCVD. According to an alternativeembodiment, the passivation stack 40 is grown by MBE. The oxide layer 42for example comprises MgO. According to an alternative embodiment, theoxide layer 42 comprises AlO_(x) or SiO_(x), or alloys thereof.According to a further alternative embodiment, the oxide layer 42comprises a gate dielectric such as for example HfO_(x), ZrO_(x), etc.The oxide layer 42 comprises an oxide surface 420 in contact with thedielectric surface 411 and a passivation insulating surface 421 oppositeto the oxide surface 420. The dielectric surface 411 and the oxidesurface 420 extend such that the oxide surface 420 is in direct contactwith the dielectric surface 411 along the full surface of the dielectricsurface 411. On FIG. 9C, a gate 30 is formed on top of the passivationstack 40 in the gate region 31. The gate 30 comprises a biasing surface300 via which a voltage bias is applied to the gate 30 and a gateinsulating surface 301 opposite to the biasing surface 300. Moreparticularly, a gate is formed in the gate region 31 on top of the oxidelayer 42, thereby defining an insulating contact interface 423 betweenthe passivation insulating surface 421 and the gate insulating surface301. The insulating contact interface 423 extends such that the gateinsulating surface 301 is in direct contact with 100% of the passivationinsulating surface 421. Ohmic contacts could be formed in a sourceregion and a drain region as described in FIG. 9C, thereby forming ametal-oxide-semiconductor field-effect transistor 1.

According to an embodiment shown in FIGS. 10A to 10C, a high electronmobility transistor according to the present invention is manufacturedas schematically illustrated by the different steps of FIGS. 10A to 10C.Components having identical reference numbers to components in FIG. 2Ato 2C and FIGS. 3A and 3B and FIGS. 4A to 4C and FIGS. 5A to 5C andFIGS. 6A to 6C and FIGS. 7A to 7C and FIGS. 8A to 8C and FIGS. 9A to 9Cperform the same function. As visible in FIG. 10A, a semiconductorstructure according to the present invention comprises a substrate 10and an epitaxial III-N semiconductor layer stack 20. The epitaxial III-Nsemiconductor layer stack 20 comprises a first active III-N layer 22 anda second active III-N layer 23, with a two dimensional Electron Gas 21between the first active III-N layer 22 and the second active III-Nlayer 23. The first active III-N layer 22 for example has a thicknesscomprised between 20 and 500 nm, preferably between 30 and 300 nm, morepreferably between 50 and 250 nm, such as for example from 100 to 150nm. The second active III-N layer 23 for example has a thicknesscomprised between 10 to 100 nm, preferably between 20 to 50 nm. Such acombination of thicknesses provides good characteristics for the activelayer, for example in terms of the 2DEG 21 obtained. The first activeIII-N layer 22 comprises nitride and one or more of B, Al, Ga, In andTl. The first active III-N layer 22 for example comprises GaN. Thesecond active III-N layer 23 comprises nitride and one or more of B, Al,Ga, In, and Tl. The second active III-N layer 23 for example comprisesAlGaN. The term AlGaN relates to a composition comprising Al, Ga and Nin any stochiometric ratio (Al_(x)Ga_(y)N) wherein x is comprisedbetween 0 and 1 and y is comprised between 0 and 1. Alternatively, thesecond active III-N layer 23 for example comprises AlN. Alternatively,the second active III-N layer 23 comprises InAlGaN. A composition suchas InAlGaN comprises In in any suitable amount. Alternatively, bothfirst active III-N layer 22 and second active III-N layer 23 compriseInAlGaN, and the second active III-N layer 23 comprises a bandgap largerthan a bandgap of the first active III-N layer 22 and wherein the secondactive III-N layer 23 comprises a polarization larger than thepolarization of the first active III-N layer 22. Alternatively, bothfirst active III-N layer 22 and second active III-N layer 23 compriseBInAlGaN, and the second active III-N layer 23 comprises a bandgaplarger than a bandgap of the first active III-N layer 22 and wherein thesecond active III-N layer 23 comprises a polarization larger than thepolarization of the first active III-N layer 22. Compositions of theactive layer may be chosen in view of characteristics to be obtained,and compositions may vary accordingly. For example, good results wereobtained with a first active III-N layer 22 comprising GaN of about 150nm thickness and a second active III-N layer 23 comprising AlGaN ofabout 20 nm thickness. A full recess 24 is formed in the second activeIII-N layer 23 in a gate region 31, thereby exposing the first activeIII-N layer 22. This can be achieved by etching in a plasma etching toolsuch as Reactive Ion Etching or RIE or preferably in an InductivelyCoupled Plasma or ICP tool. The reagent gases can be Cl₂ or BCl₃.Alternatively, a digital etching process can be used, whereasconsecutively and iteratively, first the top surface of the secondactive III-N layer is oxidized for example in O₂, O₃ or N₂O plasma,after which the formed oxide is etched away e.g. in SF₆ or CF₄ plasma.An AlN layer 45 comprising AlN is formed in the recess 24 of the secondactive III-N layer 23 as illustrated on the zoom on FIG. 10A. Accordingto an alternative embodiment, the AlN layer 45 comprising AlN is alsoformed in the recess 24 in the gate region 31 on the etched sidewalls ofthe second active III-N layer 23. The AlN layer 45 is preferably asingle monolayer of AlN. The thickness of the AlN layer 45 is preferably1 nm. An electron accepting dielectric layer 41 is formed on top of theepitaxial III-N semiconductor layer stack 20, and more particularly ontop of the second active III-N layer 23, thereby being formed op top ofthe AlN layer 45 in the recess 24 of the second active III-N layer 23.According to an alternative embodiment, a mask is deposited on top ofthe second active III-N layer 23 and the mask is etched away in the gateregion 31. The electron accepting dielectric layer 41 is then formed inthe gate region 31 as depicted in FIG. 10C. The electron acceptingdielectric layer 41 comprises a passivation surface 410 in contact withthe second active III-N layer 23 of the epitaxial III-N semiconductorlayer stack 20. The electron accepting dielectric layer 41 furthercomprises a dielectric surface 411 opposite to the passivation surface410. The second active III-N layer 23 comprises a second passivationsurface 230 in contact with the passivation surface 410 of the electronaccepting dielectric layer 41, thereby defining a passivation contactinterface 231 between the second active III-N layer 23 and the electronaccepting dielectric layer 41. The passivation contact interface 231extends such that the passivation surface 410 is in direct contact with10 to 30% of the second passivation surface in a gate region 31. Inother words, the electron accepting dielectric layer 41 is etched awayexcept in a gate region 31. The thickness of the electron acceptingdielectric layer is smaller than the depth of the recess 24 formed inthe second active III-N layer 23. The electron accepting dielectriclayer 41 comprises for example Mg_(x)Si_(1-x)N, wherein x is comprisedbetween 0.05 and 0.95. According to an alternative embodiment, theelectron accepting dielectric layer 41 comprises Mg_(y)Al_(1-y)N,wherein y is comprised between 0.05 and 0.95. According to a furtheralternative embodiment, the electron accepting dielectric layercomprises Mg_(a)Si_(z)Al_(1-a-z)N, wherein a is comprised between 0.05and 0.95 and wherein z is comprised between 0.05 and 0.95 and the a+z iscomprised between 0.1 and 1. The MgSiN or the MgAlN, or the MgSiAlN areepitaxially grown on top of the epitaxial III-N semiconductor layerstack 20, preferably on top of the second active III-N layer 23. Asshown in FIG. 10C, the passivation stack 40 further comprises an oxidelayer 42. The passivation stack 40 and more particularly the electronaccepting dielectric layer 41 and the oxide layer 42 are for examplegrown by MOCVD. According to an alternative embodiment, the passivationstack 40 is grown by MBE. The oxide layer 42 for example comprises MgO.According to an alternative embodiment, the oxide layer 42 comprisesAlO_(x) or SiO_(x), or alloys thereof. According to a furtheralternative embodiment, the oxide layer 42 comprises a gate dielectricsuch as for example HfO_(x), ZrO_(x), etc. The oxide layer 42 alsoextends in the recess 24 formed in the second active III-N layer 23. Theoxide layer 42 comprises an oxide surface 420 in contact with thedielectric surface 411 and a passivation insulating surface 421 oppositeto the oxide surface 420. The dielectric surface 411 and the oxidesurface 420 extend such that the oxide surface 420 is in direct contactwith the dielectric surface 411 along the full surface of the dielectricsurface 411. On FIG. 10C, a gate 30 is formed on top of the passivationstack 40 in the gate region 31. The gate 30 comprises a biasing surface300 via which a voltage bias is applied to the gate 30 and a gateinsulating surface 301 opposite to the biasing surface 300. Moreparticularly, a gate is formed in the gate region 31 on top of the oxidelayer 42, thereby defining an insulating contact interface 423 betweenthe passivation insulating surface 421 and the gate insulating surface301. The insulating contact interface 423 extends such that the gateinsulating surface 301 is in direct contact with 100% of the passivationinsulating surface 421. Ohmic contacts could be formed in a sourceregion and a drain region as described in FIG. 10C, thereby forming ametal-oxide-semiconductor field-effect transistor 1.

According to an embodiment shown in FIGS. 11A to 11C, a high electronmobility transistor according to the present invention is manufacturedas schematically illustrated by the different steps of FIGS. 11A to 11C.Components having identical reference numbers to components in FIG. 2Ato 2C and FIGS. 3A and 3B and FIGS. 4A to 4C and FIGS. 5A to 5C andFIGS. 6A to 6C and FIGS. 7A to 7C and FIGS. 8A to 8C and FIGS. 9A to 9Cand FIGS. 10A to 10C perform the same function. As visible in FIG. 11A,a semiconductor structure according to the present invention comprises asubstrate 10 and an epitaxial III-N semiconductor layer stack 20. Theepitaxial III-N semiconductor layer stack 20 comprises a first activeIII-N layer 22 and a second active III-N layer 23, with a twodimensional Electron Gas 21 between the first active III-N layer 22 andthe second active III-N layer 23. The first active III-N layer 22 forexample has a thickness comprised between 20 and 500 nm, preferablybetween 30 and 300 nm, more preferably between 50 and 250 nm, such asfor example from 100 to 150 nm. The second active III-N layer 23 forexample has a thickness comprised between 10 to 100 nm, preferablybetween 20 to 50 nm. Such a combination of thicknesses provides goodcharacteristics for the active layer, for example in terms of the 2DEG21 obtained. The first active III-N layer 22 comprises nitride and oneor more of B, Al, Ga, In and Tl. The first active III-N layer 22 forexample comprises GaN. The second active III-N layer 23 comprisesnitride and one or more of B, Al, Ga, In, and Tl. The second activeIII-N layer 23 for example comprises AlGaN. The term AlGaN relates to acomposition comprising Al, Ga and N in any stochiometric ratio(Al_(x)Ga_(y)N) wherein x is comprised between 0 and 1 and y iscomprised between 0 and 1. Alternatively, the second active III-N layer23 for example comprises AlN. Alternatively, the second active III-Nlayer 23 comprises InAlGaN. A composition such as InAlGaN comprises Inin any suitable amount. Alternatively, both first active III-N layer 22and second active III-N layer 23 comprise InAlGaN, and the second activeIII-N layer 23 comprises a bandgap larger than a bandgap of the firstactive III-N layer 22 and wherein the second active III-N layer 23comprises a polarization larger than the polarization of the firstactive III-N layer 22. Alternatively, both first active III-N layer 22and second active III-N layer 23 comprise BInAlGaN, and the secondactive III-N layer 23 comprises a bandgap larger than a bandgap of thefirst active III-N layer 22 and wherein the second active III-N layer 23comprises a polarization larger than the polarization of the firstactive III-N layer 22. Compositions of the active layer may be chosen inview of characteristics to be obtained, and compositions may varyaccordingly. For example, good results were obtained with a first activeIII-N layer 22 comprising GaN of about 150 nm thickness and a secondactive III-N layer 23 comprising AlGaN of about 20 nm thickness. A fullrecess 24 is formed in the second active III-N layer 23 in a gate region31, thereby exposing the first active III-N layer 22. This can beachieved by etching in a plasma etching tool such as Reactive IonEtching or RIE or preferably in an Inductively Coupled Plasma or ICPtool. The reagent gases can be Cl₂ or BCl₃. Alternatively, a digitaletching process can be used, whereas consecutively and iteratively,first the top surface of the second active III-N layer is oxidized forexample in O₂, O₃ or N₂O plasma, after which the formed oxide is etchedaway e.g. in SF₆ or CF₄ plasma. An AlN layer 45 comprising AlN is formedin the recess 24 of the second active III-N layer 23. According to analternative embodiment, the AlN layer 45 comprising AlN is also formedin the recess 24 in the gate region 31 on the etched sidewalls of thesecond active III-N layer 23. The AlN layer 45 is preferably a singlemonolayer of AlN. The thickness of the AlN layer 45 is preferably 1 nm.An electron accepting dielectric layer 41 is formed on top of theepitaxial semiconductor layer stack 20, and more particularly on top ofthe second active layer 23, thereby being formed op top of the AlN layer45 in the recess 24 of the second active layer 23. According to analternative embodiment, a mask is deposited on top of the second activelayer 23 and the mask is etched away in the gate region 31. The electronaccepting dielectric layer 41 is then formed in the gate region 31 asdepicted in FIG. 11B. The electron accepting dielectric layer 41comprises a passivation surface 410 in contact with the second activelayer 23 of the epitaxial semiconductor layer stack 20. The electronaccepting dielectric layer 41 further comprises a dielectric surface 411opposite to the passivation surface 410. The second active III-N layer23 comprises a second passivation surface 230 in contact with thepassivation surface 410 of the electron accepting dielectric layer 41,thereby defining a passivation contact interface 231 between the secondactive layer 23 and the electron accepting dielectric layer 41. Thepassivation contact interface 231 extends such that the passivationsurface 410 is in direct contact with 10 to 30% of the secondpassivation surface in a gate region 31. In other words, the electronaccepting dielectric layer 41 is etched away except in a gate region 31.The thickness of the electron accepting dielectric layer is larger thanthe depth of the recess 24 formed in the second active layer 23. Theelectron accepting dielectric layer 41 comprises for exampleMg_(x)Si_(1-x)N, wherein x is comprised between 0.05 and 0.95. Accordingto an alternative embodiment, the electron accepting dielectric layer 41comprises Mg_(y)Al_(1-y)N, wherein y is comprised between 0.05 and 0.95.According to a further alternative embodiment, the electron acceptingdielectric layer comprises Mg_(a)Si_(z)Al_(1-a-z)N, wherein a iscomprised between 0.05 and 0.95 and wherein z is comprised between 0.05and 0.95 and the a+z is comprised between 0.1 and 1. The MgSiN or theMgAlN, or the MgSiAlN are epitaxially grown on top of the epitaxialIII-N semiconductor layer stack 20, preferably on top of the secondactive III-N layer 23. As shown in FIG. 11C, the passivation stack 40further comprises an oxide layer 42. The passivation stack 40 and moreparticularly the electron accepting dielectric layer 41 and the oxidelayer 42 are for example grown by MOCVD. According to an alternativeembodiment, the passivation stack 40 is grown by MBE. The oxide layer 42for example comprises MgO. According to an alternative embodiment, theoxide layer 42 comprises AlO_(x) or SiO_(x), or alloys thereof.According to a further alternative embodiment, the oxide layer 42comprises a gate dielectric such as for example HfO_(x), ZrO_(x), etc.The oxide layer 42 comprises an oxide surface 420 in contact with thedielectric surface 411 and a passivation insulating surface 421 oppositeto the oxide surface 420. The dielectric surface 411 and the oxidesurface 420 extend such that the oxide surface 420 is in direct contactwith the dielectric surface 411 along the full surface of the dielectricsurface 411. On FIG. 11C, a gate 30 is formed on top of the passivationstack 40 in the gate region 31. The gate 30 comprises a biasing surface300 via which a voltage bias is applied to the gate 30 and a gateinsulating surface 301 opposite to the biasing surface 300. Moreparticularly, a gate is formed in the gate region 31 on top of the oxidelayer 42, thereby defining an insulating contact interface 423 betweenthe passivation insulating surface 421 and the gate insulating surface301. The insulating contact interface 423 extends such that the gateinsulating surface 301 is in direct contact with 100% of the passivationinsulating surface 421. Ohmic contacts could be formed in a sourceregion and a drain region as described in FIG. 11C, thereby forming ametal-oxide-semiconductor field-effect transistor 1.

FIG. 12 schematically illustrates the steps of a manufacturing method ofa high electron mobility transistor according to the present invention.In step 101, a substrate 10 is provided. In step 102, an epitaxial III-Nsemiconductor layer stack 20 is consequently provided on top of thesubstrate 10. The epitaxial III-N semiconductor layer stack 20 comprisesan active layer comprising a first active III-N layer, a second activeIII-N layer on top of the first active III-N layer, wherein the secondactive III-N layer comprises a recess 24, and wherein a two dimensionalElectron Gas between the first active III-N layer and the second activeIII-N layer. In step 103, a passivation stack 40 is then provided on topof the epitaxial III-N semiconductor layer stack 20. The passivationstack 40 comprises an electron accepting dielectric layer 41. Theelectron accepting dielectric layer 41 comprises magnesium nitride dopedwith silicon and/or aluminum. The electron accepting dielectric layer 41extends in the recess 24. Finally, in step 104, a gate 30 is provided ontop of the electron accepting dielectric layer 41 in a gate region 31.

Although the present invention has been illustrated by reference tospecific embodiments, it will be apparent to those skilled in the artthat the invention is not limited to the details of the foregoingillustrative embodiments, and that the present invention may be embodiedwith various changes and modifications without departing from the scopethereof. The present embodiments are therefore to be considered in allrespects as illustrative and not restrictive, the scope of the inventionbeing indicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.In other words, it is contemplated to cover any and all modifications,variations or equivalents that fall within the scope of the basicunderlying principles and whose essential attributes are claimed in thispatent application. It will furthermore be understood by the reader ofthis patent application that the words “comprising” or “comprise” do notexclude other elements or steps, that the words “a” or “an” do notexclude a plurality, and that a single element, such as a computersystem, a processor, or another integrated unit may fulfil the functionsof several means recited in the claims. Any reference signs in theclaims shall not be construed as limiting the respective claimsconcerned. The terms “first”, “second”, third”, “a”, “b”, “c”, and thelike, when used in the description or in the claims are introduced todistinguish between similar elements or steps and are not necessarilydescribing a sequential or chronological order. Similarly, the terms“top”, “bottom”, “over”, “under”, and the like are introduced fordescriptive purposes and not necessarily to denote relative positions.It is to be understood that the terms so used are interchangeable underappropriate circumstances and embodiments of the invention are capableof operating according to the present invention in other sequences, orin orientations different from the one(s) described or illustratedabove.

The invention claimed is:
 1. A high electron mobility transistor (HEMT) for analog applications, said high electron mobility transistor comprising: a substrate; an epitaxial III-N semiconductor layer stack on top of said substrate, said epitaxial III-N semiconductor layer stack comprising an active layer, said active layer comprising: a first active III-N layer; and a second active III-N layer comprising a recess in a gate region; with a two-dimensional Electron Gas between said first active III-N layer and said second active III-N layer; a gate on top of said epitaxial III-N semiconductor layer stack and in said gate region; and a passivation stack between said epitaxial III-N semiconductor layer stack and said gate, wherein said passivation stack comprises an electron accepting dielectric layer adapted to deplete said two-dimensional Electron Gas when said gate is not biased; wherein said electron accepting dielectric layer extends in said recess; wherein said electron accepting dielectric layer comprises magnesium nitride doped with silicon and/or aluminum; and wherein said passivation stack further comprises an oxide layer.
 2. The HEMT according to claim 1, wherein said electron accepting dielectric layer comprises one or more of the following: MgSiN; MgAlN; MgSiAlN.
 3. The HEMT according to claim 1, wherein said electron accepting dielectric layer comprises one or more of the following: Mg_(x)Si_(1-x)N, wherein x is comprised between 0.05 and 0.95; Mg_(y)Al_(1-y)N, wherein y is comprised between 0.05 and 0.95; Mg_(a)Si_(z)Al_(1-a-z)N, wherein a is comprised between 0.05 and 0.95 and wherein z is comprised between 0.05 and 0.95 and wherein a+z is comprised between 0.1 and
 1. 4. The HEMT according to claim 1, wherein said electron accepting dielectric layer is epitaxially grown on top of said epitaxial III-N semiconductor layer stack.
 5. The HEMT according to claim 1, wherein said oxide layer comprises MgO.
 6. The HEMT according to claim 1, wherein: said electron accepting dielectric layer comprises a passivation surface in contact with said epitaxial III-N semiconductor layer stack and a dielectric surface opposite to said passivation surface; and said second active III-N layer comprises a second passivation surface in contact with said passivation surface of said electron accepting dielectric layer, thereby defining a passivation contact interface between said second active III-N layer and said electron accepting dielectric layer.
 7. The HEMT according to claim 6, wherein said passivation contact interface extends such that said passivation surface is in direct contact with said second passivation surface along the full surface of said second passivation surface.
 8. The HEMT according to claim 6, wherein: said passivation contact interface extends such that said passivation surface is in direct contact with 10 to 30% of said second passivation surface in a gate region; and said passivation stack further comprises two electron donating dielectric layers formed on top of said second active III-N layer and on both sides of said electron accepting dielectric layer such that each of two electron donating dielectric layers comprises a III-N contact surface in direct contact with said second active III-N layer.
 9. The HEMT according to claim 6, wherein said recess in said gate region extends completely through said second active III-N layer thereby exposing said first active III-N layer.
 10. The HEMT according to claim 9, wherein said electron accepting dielectric layer extends in said recess such that said passivation surface is in direct contact with said first active III-N layer in said recess.
 11. A high electron mobility transistor (HEMT) for analog applications, said high electron mobility transistor comprising: a substrate; an epitaxial III-N semiconductor layer stack on top of said substrate, said epitaxial III-N semiconductor layer stack comprising an active layer, said active layer comprising: a first active III-N layer; and a second active III-N layer comprising a recess in a gate region; with a two-dimensional Electron Gas between said first active III-N layer and said second active III-N layer; a gate on top of said epitaxial III-N semiconductor layer stack and in said gate region; and a passivation stack between said epitaxial III-N semiconductor layer stack and said gate, wherein said passivation stack comprises an electron accepting dielectric layer adapted to deplete said two-dimensional Electron Gas when said gate is not biased; wherein said electron accepting dielectric layer extends in said recess; wherein said electron accepting dielectric layer comprises magnesium nitride doped with silicon and/or aluminum; wherein said electron accepting dielectric layer comprises a passivation surface in contact with said epitaxial III-N semiconductor layer stack and a dielectric surface opposite to said passivation surface; wherein said second active III-N layer comprises a second passivation surface in contact with said passivation surface of said electron accepting dielectric layer, thereby defining a passivation contact interface between said second active III-N layer and said electron accepting dielectric layer; wherein said recess in said gate region extends completely through said second active III-N layer thereby exposing said first active III-N layer; wherein said passivation stack further comprises an AlN layer such that said AlN layer is in direct contact with said first active III-N layer in said recess; and wherein said electron accepting dielectric layer extends in said recess on top of said AlN layer.
 12. A method for manufacturing a high electron mobility transistor, said method comprising the steps of: providing a substrate; providing an epitaxial III-N semiconductor layer stack on top of said substrate, wherein said providing said epitaxial III-N semiconductor layer stack comprises providing an active layer comprising: a first active III-N layer; and a second active III-N layer; thereby forming a two-dimensional Electron Gas between said first active III-N layer and said second active III-N layer; forming a recess in the second active III-N layer in a gate region; providing a passivation stack on top of said epitaxial III-N semiconductor layer stack, wherein said passivation stack comprises an electron accepting dielectric layer; and providing a gate on top of said electron accepting dielectric layer in a gate region such that said electron accepting dielectric layer depletes said two-dimensional Electron Gas when said gate is not biased; wherein said electron accepting dielectric layer extends in said recess and wherein said electron accepting dielectric layer comprises magnesium nitride doped with silicon and/or aluminum, and wherein said providing said electron accepting dielectric layer corresponds to epitaxially growing said electron accepting dielectric layer.
 13. The method according to claim 12, wherein said method further comprises the steps of: etching said passivation stack in a source region and a drain region; and forming an ohmic contact respectively in said source region and said drain region. 